Move catch_command_errors and catch_command_errors_const to main.c

We'll need to add error handling code to commands run before the event
loop starts (commands in .gdbinit, -ex commands, etc.).  Turns out
those are run through catch_command_errors, and, catch_command_errors
is used nowhere else.  Move it (and the _const variant) to main.c, so
that we can further specialize it freely.

gdb/
2014-07-14  Pedro Alves  <palves@redhat.com>

	* exceptions.c (catch_command_errors, catch_command_errors_const):
	Moved to main.c.
	* exceptions.h (catch_command_errors_ftype)
	(catch_command_errors_const_ftype): Moved to main.c.
	(catch_command_errors, catch_command_errors_const): Delete
	declarations.
	* main.c (catch_command_errors_ftype)
	(catch_command_errors_const_ftype): Moved here from exceptions.h.
	(catch_command_errors, catch_command_errors_const)): Moved here
	from exceptions.c and make static.