epg: Collect output lines sent to stderr

* epg.el (epg-context): Add new slot ERROR-OUTPUT.
(epg-error-output): New buffer-local variable.
(epg--start): Initialize epg-error-output.
(epg--process-filter): Record output lines sent to stderr, in
epg-error-output.
(epg-wait-for-completion): Copy epg-error-output to ERROR-OUTPUT
slot of context.
* epa-file.el (epa-file-insert-file-contents): On error, display
output sent to stderr.
(epa-file-write-region): Likewise.