DomElement: Throw std::runtime_error if data is not XML

Sanity check to catch project files from Eagle v5 or older, which
were binary and thus not supported by this parser.

Author: ubruhin

parseagle/common/domelement.cpp

@@ -53,6 +53,13 @@ DomElement DomElement::parse(QXmlStreamReader& reader)
 
 DomElement DomElement::parse(QByteArray data)
 {
+    // Sanity check that no Eagle v5 (binary) project is imported.
+    if (!data.contains("<?xml")) {
+        throw std::runtime_error("File does not seem to contain valid XML. "
+                                 "Please note that only projects from EAGLE v6 "
+                                 "or later are supported.");
+    }
+
     // Workaround for garbage in some Eagle XML files, see
     // https://gitlab.com/kicad/code/kicad/-/work_items/11008
     data.replace("\x0c", "");