Throw and error, do not just log an error, if provided schema is not readable

I think current behavior violates the principle of the least surprise: I ran
validation while providing custom schema source, it executed with many warnings
and errors reported to the screen. It is only after scrolling far up, I saw
that actually my specification was wrong and operation resorted to built-in schema.

Author: yarikoptic

src/setup/loadSchema.ts

@@ -31,10 +31,10 @@ export async function loadSchema(version?: string): Promise<Schema> {
         objectPathHandler,
       ) as Schema
     } catch (error) {
-      // No network access or other errors
+      // If a custom schema URL was explicitly provided, fail rather than falling back
       console.error(error)
-      console.error(
-        `Warning, could not load schema from ${schemaUrl}, falling back to internal version`,
+      throw new Error(
+        `Failed to load schema from ${schemaUrl}: ${error instanceof Error ? error.message : String(error)}`,
       )
     }
   }