Signal an error in the user clicks "cancel" when signing in epg

* lisp/epg.el (epg-sign-string): If the user clicks "cancel" on
the pinentry, then we don't have an error from gpg(sm), but
instead nothing (bug#39058).  Signal an error in that case.
1 job for master in 56 minutes and 45 seconds (queued for 1 second)
Status Job ID Name Coverage
failed #6800


Name Stage Failure
test-all Test
Makefile:318: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:293: recipe for target 'check-expensive' failed
make[1]: Leaving directory '/builds/emacs/emacs/test'
make[2]: *** [check-doit] Error 1
make[1]: *** [check-expensive] Error 2
make: *** [check-expensive] Error 2
Makefile:959: recipe for target 'check-expensive' failed
ERROR: Job failed: exit code 1