Give useful messages for warn_if_fail() etc.
Submitted by Sam Thursfield
Link to original bug (#664108)
Description
When a warn_if_fail() assertion with more than one condition fails, its error message is meaningless:
** (process:8198): CRITICAL **: vala_main: assertion `_tmp4' failed
Instead of using g_warn_if_fail() internally we could generate a message from the actual Vala code