Catch most exceptions in main() to report exceptions uncaught by Squid. This

Catch most exceptions in main() to report exceptions uncaught by Squid.  This
is for last resort reporting only -- the program would exit anyway (usually
with less information) if we did not catch these.

The code re-throws caught exceptions to reduce side effects of catching it,
just in case. May need more work depending on how compilers handle rethrowing.

These changes were inspired by eCAP.