bug#6963: More usecases, patch attached

[Wolfgang Schnerring]

emacsclient is such a usefull tool. But that it does not signal error
situations via its exit status severely hinders its usage in shell
scripts or in integration with other programs.

For example, I was bit by this rather badly when I tried to use
emacsclient to start an ediff session, to use as a merge tool for the
Mercurial SCM.
I may have made a typo in configuring said ediff command or for
whatever other reason, Emacs returned an error. But Mercurial was
blissfully unaware of this, since emacsclient returns 0 no matter
what. End result: Mercurial committed a broken merge that I had to
spend at least two hours some days later to debug.

I've attached a patch that makes emacsclient exit with nonzero status
when it receives an error message from Emacs -- it already *prints*
"*ERROR*" in these cases, so I feel this makes a lot of sense.

Thanks,
Wolfgang

emacsclient-exitstatus.patch
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