Handle auth-source-search failures in open-network-stream

If the user cancels the gpg decryption pop-up, auth-source-search
fails *and* epa pops up an error buffer.  Fix epa to allow suppressing
that, and ignore errors returned from auth-source-search.

* lisp/epa.el (epa-suppress-error-buffer): New defvar.  Bind non-nil
to stop epa popping up an error buffer.

* lisp/net/network-stream.el: require epa when byte-compiling.
(network-stream-certificate): ignore errors when calling
auth-source-search, and suppress the epa error buffer.
1 job for master in 62 minutes and 20 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #4336
test-all

01:02:20

 
Name Stage Failure
failed
test-all Test
Makefile:319: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:294: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:959: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1