[ZEPPELIN-6241] Fail fast when default web app context fails to initialize due to missing resources

### What is this PR for?
Currently, the server keeps running even when all web app contexts fail to initialize due to missing files or directories for web resources.

In my opinion, if the web resource path for the default web app context does not exist, it would be better to shut down the server immediately, since the context initialization will fail anyway. In such cases, other essential features like REST APIs and WebSocket communication also won’t work properly, so keeping the server running doesn’t seem meaningful.

The absence of non-default web resources, however, seems generally acceptable. So this PR ensures that we only fail fast when the default web app context is missing its required resources.

### What type of PR is it?
Improvement

### What is the Jira issue?
- https://issues.apache.org/jira/browse/ZEPPELIN-6241

### How should this be tested?
- Start the server with the default web app directory intentionally missing.
- Verify that the server fails to start and exit immediately.
- Ensure that non-default apps can still be missing without preventing startup

### Questions:
* Does the license files need to update? No
* Is there breaking changes for older versions? No
* Does this needs documentation? No

Closes #4969 from tbonelee/fail-fast.

Signed-off-by: Philipp Dallig <philipp.dallig@gmail.com>

Author: tbonelee

bin/common.sh

@@ -46,7 +46,9 @@ if [[ -z "${ZEPPELIN_WAR}" ]]; then
     export ZEPPELIN_WAR="${ZEPPELIN_HOME}/zeppelin-web/dist"
   else
     ZEPPELIN_WAR=$(find -L "${ZEPPELIN_HOME}" -name "zeppelin-web-[0-9]*.war")
-    export ZEPPELIN_WAR
+    if [[ -n "${ZEPPELIN_WAR}" ]]; then
+      export ZEPPELIN_WAR
+    fi
   fi
 fi
 
@@ -55,7 +57,9 @@ if [[ -z "${ZEPPELIN_ANGULAR_WAR}" ]]; then
     export ZEPPELIN_ANGULAR_WAR="${ZEPPELIN_HOME}/zeppelin-web-angular/dist/zeppelin"
   else
     ZEPPELIN_ANGULAR_WAR=$(find -L "${ZEPPELIN_HOME}" -name "zeppelin-web-angular*.war")
-    export ZEPPELIN_ANGULAR_WAR
+    if [[ -n "${ZEPPELIN_ANGULAR_WAR}" ]]; then
+      export ZEPPELIN_ANGULAR_WAR
+    fi
   fi
 fi
 

zeppelin-server/src/main/java/org/apache/zeppelin/server/ZeppelinServer.java

@@ -221,19 +221,14 @@ protected void configure() {
         });
 
     // Multiple Web UI
-    String classicUiWebAppContextPath;
-    String newUiWebAppContextPath;
-    if (isNewUiDefault(zConf)) {
-      classicUiWebAppContextPath = NON_DEFAULT_CLASSIC_UI_WEB_APP_CONTEXT_PATH;
-      newUiWebAppContextPath = zConf.getServerContextPath();
-    } else {
-      classicUiWebAppContextPath = zConf.getServerContextPath();
-      newUiWebAppContextPath = NON_DEFAULT_NEW_UI_WEB_APP_CONTEXT_PATH;
-    }
+    String newUiWebAppContextPath = isNewUiDefault(zConf) ?  zConf.getServerContextPath() : NON_DEFAULT_NEW_UI_WEB_APP_CONTEXT_PATH;
+    boolean newUiWebAppShouldExist = isNewUiDefault(zConf);
+    String classicUiWebAppContextPath = !isNewUiDefault(zConf) ? zConf.getServerContextPath() : NON_DEFAULT_CLASSIC_UI_WEB_APP_CONTEXT_PATH;
+    boolean classicUiWebAppShouldExist = !isNewUiDefault(zConf);
     final WebAppContext newUiWebApp = setupWebAppContext(contexts, zConf, zConf.getString(ConfVars.ZEPPELIN_ANGULAR_WAR),
-        newUiWebAppContextPath);
+        newUiWebAppContextPath, newUiWebAppShouldExist);
     final WebAppContext classicUiWebApp = setupWebAppContext(contexts, zConf, zConf.getString(ConfVars.ZEPPELIN_WAR),
-        classicUiWebAppContextPath);
+        classicUiWebAppContextPath, classicUiWebAppShouldExist);
 
     initWebApp(newUiWebApp);
     initWebApp(classicUiWebApp);
@@ -276,7 +271,7 @@ protected void configure() {
       jettyWebServer.start(); // Instantiates ZeppelinServer
     } catch (Exception e) {
       LOGGER.error("Error while running jettyServer", e);
-      System.exit(-1);
+      shutdown(-1);
     }
 
     LOGGER.info("Done, zeppelin server started");
@@ -287,7 +282,7 @@ protected void configure() {
       }
       if (!errorDatas.isEmpty()) {
         LOGGER.error("{} error(s) while starting - Termination", errorDatas.size());
-        System.exit(-1);
+        shutdown(-1);
       }
     } catch (InterruptedException e) {
       // Many fast unit tests interrupt the Zeppelin server at this point
@@ -586,10 +581,12 @@ private static void setupHealthCheckContextHandler(WebAppContext webapp) {
   }
 
   private static WebAppContext setupWebAppContext(
-      ContextHandlerCollection contexts, ZeppelinConfiguration zConf, String warPath, String contextPath) {
+      ContextHandlerCollection contexts, ZeppelinConfiguration zConf, String warPath, String contextPath, boolean shouldExist) {
     WebAppContext webApp = new WebAppContext();
     webApp.setContextPath(contextPath);
     LOGGER.info("warPath is: {}", warPath);
+    LOGGER.info("The file or directory for the warPath should exist: {}", shouldExist);
+    webApp.setThrowUnavailableOnStartupException(shouldExist);
     File warFile = new File(warPath);
     if (warFile.isDirectory()) {
       // Development mode, read from FS