Validate upfront that an interface is compatible with our Java backend,

emit diagnostics if it is not. Also cleans up all other error messages to
clearly indicate an error condition.

Bug: 30876839
Change-Id: I18bcd723107ab93abcad38c976f3c38dda60a743
diff --git a/Method.h b/Method.h
index 11caf3e..3ba9c52 100644
--- a/Method.h
+++ b/Method.h
@@ -35,6 +35,8 @@
 
     void dumpAnnotations(Formatter &out) const;
 
+    bool isJavaCompatible() const;
+
 private:
     std::string mName;
     std::vector<TypedVar *> *mArgs;
@@ -51,6 +53,8 @@
     std::string name() const;
     const Type &type() const;
 
+    bool isJavaCompatible() const;
+
 private:
     std::string mName;
     Type *mType;