David Brownell <david-b@pacbell.net> Add "cortex_m3 vector_catch" command and docs...

David Brownell <david-b@pacbell.net> Add "cortex_m3 vector_catch" command and docs.  One minor
issue with this is that the core debug support uses this
mechanism, then trashes its state over reset.  Users can
Work around that (for now) by re-assigning the desired
config after reset.

Also fixes "target halted due to target-not-halted" goof.
When we can't describe the reason using OpenOCD's limited
vocabulary, say "reason undefined" instead of saying it's
not halted.

git-svn-id: svn://svn.berlios.de/openocd/trunk@2588 b42882b7-edfa-0310-969c-e2dbd0fdcd60