Update enter_chroot.sh to exit with error messages when mount/unmount fails.

Update enter_chroot.sh to exit with error messages when mount/unmount fails.

This is necessary for two reasons:
   1) It's nice to get an error message when mount/unmount fails
   2) set -e mode doesn't have any effect when you're in a subshell

Note that these mount/unmount failures do happen regularly in development,
so folks who depended on mount/umount failing silently will no longer be
able to rely on this and will have to kill the mounts manually.

Also fix subtle bugs in regexes for matching mount paths. (E.g. where the
regex for $HOME/chroot also matches $HOME/chroot2).

TEST=Tested mount/unmount with concurrently open shells.
     Tested mount/unmount when mount is being used by a process but the
     lock file does not reflect this.
BUG=none

[adlr, 2010-03-23 21:17:57]

LGTM

[djmm, 2010-03-23 21:20:51]

LGTM

[djmm, 2010-03-23 21:23:01]

Can you add BUG=1788 to this.