Fixes for three related bugs, including errors that caused a script to
be ignored without printing an error message.  The key problem was a bad
interaction between syntax warnings and syntax errors.  If an
exception was already set when a warning was issued, the warning could
clobber the exception.

The PyErr_Occurred() check in issue_warning() isn't entirely
satisfying (the caller should know whether there was already an
error), but a better solution isn't immediately obvious.

Bug fix candidate.