I decided to null out EG(exception) early here, which means only
the exception from the dtor / ref assign is preserved, and the
previous exception is not chained in. This is more robust, and
I don't think this situation is common enough to be bothered about
the precise behavior.
This implements a reduced variant of #1226 with just the following
change:
-Fatal error: Uncaught exception 'EngineException' with message 'Call to private method foo::bar() from context ''' in %s:%d
+Fatal error: Uncaught EngineException: Call to private method foo::bar() from context '' in %s:%d
The '' wrapper around messages is very weird if the exception
message itself contains ''. Futhermore having the message wrapped
in '' doesn't work for the "and defined" suffix of
TypeExceptions.
