Fix swallowed "Quit" when inserting breakpoints

Fix swallowed "Quit" when inserting breakpoints

If GDB is inserting a breakpoint and you type Ctrl-C at the exact
"right" time, you'll hit a QUIT call in target_read, and the
breakpoint insertion is cancelled.  However, the related TRY/CATCH
code in insert_bp_location does:

 		  CATCH (e, RETURN_MASK_ALL)
 		    {
		      bp_err = e.error;
		      bp_err_message = e.message;
		    }

The problem with that is that a RETURN_QUIT exception has e.error ==
0, which means that further below, in the places that check for error
with:

      if (bp_err != GDB_NO_ERROR)

because GDB_NO_ERROR == 0, GDB continues as if the breakpoint was
inserted succesfully, and resumes the inferior.  Since the breakpoint
wasn't inserted the inferior runs free, out of our control...

Fix this by having insert_bp_location store a copy of the whole
exception instead of just a error/message parts, and then checking
"gdb_exception::reason" instead.

This was exposed by the new gdb.base/bp-cmds-continue-ctrl-c.exp
testcase added later in the series.

gdb/ChangeLog:
2017-11-16  Pedro Alves  <palves@redhat.com>

	* breakpoint.c (insert_bp_location): Replace bp_err and
	bp_err_message locals by a gdb_exception local.