fix: minor error message wording

Author: Julia Pham

src/main/java/de/vill/exception/ErrorReport.java

@@ -67,7 +67,7 @@ public String toString() {
             sb.append("\n  Field: ").append(field);
         }
         if (cause != null) {
-            sb.append("\n  Cause: ").append(cause);
+            sb.append("\n  Possible cause: ").append(cause);
         }
         if (hint != null) {
             sb.append("\n  Hint: ").append(hint);

src/main/java/de/vill/main/UVLErrorListener.java

@@ -66,7 +66,7 @@ private ErrorReport translateToReport(String message, Object offendingSymbol, Re
                     "Wrong feature name: " + extra)
                     .line(line).charPosition(charPosition)
                     .reference(m.group(1))
-                    .cause("Feature names can not start with a number.")
+                    .cause("Invalid feature name. Feature names can not start with a number.")
                     .hint("Rename the feature.")
                     .build();
             }
@@ -85,7 +85,7 @@ private ErrorReport translateToReport(String message, Object offendingSymbol, Re
             String found = tokenToReadable(m.group(1));
             String expected = simplifySet(m.group(2));
 
-            // Sonderfall: nach Gruppierung ohne Features
+            // Case: group keywords without features/children
             if (expected.equals("an indentation") && (found.equals("a new line") || found.contains("'mandatory'") || found.contains("'optional'")|| found.contains("'alternative'") || found.contains("'or'"))) {
                 return new ErrorReport.Builder(ErrorCategory.SYNTAX,
                         "Missing features after group type")

src/main/java/de/vill/main/UVLListener.java

@@ -110,8 +110,8 @@ public void exitIncludeLine(UVLJavaParser.IncludeLineContext ctx) {
                     .line(line).charPosition(charPos)
                     .field(ErrorField.LANGUAGE_LEVEL)
                     .reference(ctx.languageLevel().getText())
-                    .cause("Language level imports must have the format 'MajorLevel' or 'MajorLevel.MinorLevel'.")
-                    .hint("Use a valid language level format, e.g. 'Arithmetic-Level' or 'Arithmetic-Level.Aggregate-Function'.")
+                    .cause("Invalid language level import.")
+                    .hint("Use a valid language level format.")
                     .build()));
         }
     }
@@ -233,8 +233,6 @@ public void enterFeature(UVLJavaParser.FeatureContext ctx) {
         int line = ctx.getStart().getLine();
         int charPos = ctx.getStart().getCharPositionInLine();
 
-        // Group keywords cannot be used as feature names — this typically
-        // indicates a wrong indentation rather than an intentional feature name.
         if (GROUP_KEYWORDS.contains(featureReference.toLowerCase())) {
             errorList.add(new ParseError(new ErrorReport.Builder(ErrorCategory.SYNTAX,
                     "'" + featureReference + "' is a reserved group keyword and cannot be used as a feature name")