Make exception classes have clearer messages

Include the type of exception in the string generated by each
subclass. Also simplify the constructs to what is needed.
diff --git a/common/rfb/CSecurityTLS.cxx b/common/rfb/CSecurityTLS.cxx
index 8116e9c..9a698f0 100644
--- a/common/rfb/CSecurityTLS.cxx
+++ b/common/rfb/CSecurityTLS.cxx
@@ -153,7 +153,7 @@
       if (result == secResultFailed || result == secResultTooMany)
         reason.buf = is->readString();
       else
-        reason.buf = strDup("Authentication failure (protocol error)");
+        reason.buf = strDup("protocol error");
       throw AuthFailureException(reason.buf);
     }