Only catch RaiseException for ExceptionOperations#messageToString

* Use messageFieldToString() for RaiseException#getMessage() as the
  context might not be entered.

Author: eregon

src/main/java/org/truffleruby/core/exception/ExceptionOperations.java

@@ -97,7 +97,7 @@ public static String getMessage(Throwable throwable) {
     }
 
     @TruffleBoundary
-    private static String messageFieldToString(RubyException exception) {
+    public static String messageFieldToString(RubyException exception) {
         Object message = exception.message;
         RubyStringLibrary strings = RubyStringLibrary.getUncached();
         if (message == null || message == Nil.INSTANCE) {
@@ -115,7 +115,7 @@ public static String messageToString(RubyException exception) {
         Object messageObject = null;
         try {
             messageObject = DispatchNode.getUncached().call(exception, "message");
-        } catch (Throwable e) {
+        } catch (RaiseException e) {
             // Fall back to the internal message field
         }
         if (messageObject != null && RubyStringLibrary.getUncached().isRubyString(messageObject)) {

src/main/java/org/truffleruby/language/control/RaiseException.java

@@ -53,7 +53,7 @@ public RubyException getException() {
 
     @Override
     public String getMessage() {
-        return ExceptionOperations.messageToString(exception);
+        return ExceptionOperations.messageFieldToString(exception);
     }
 
 }