vala: Improve error output of mismatching overriding methods
vala: Improve error output of mismatching overriding methods

Include the prototype-string of base-method for easier error finding.

frida/vala,GNOME/vala,frida/vala,frida/vala,frida/vala,GNOME/vala,GNOME/vala,GNOME/vala,frida/vala