Silently failing to sign zip

[tomasdev]

If a certificate pem file is not found, it doesn't complain at all. It creates the zip without the signature file.

Users should be able to see something like:

[Error: Error opening signer certificate .../keys/com.domain.demo.pem
65898:error:02001002:system library:fopen:No such file or directory:/SourceCache/OpenSSL098/OpenSSL098-52.8.1/src/crypto/bio/bss_file.c:356:fopen('.../keys/com.domain.demo.pem','r')
65898:error:20074002:BIO routines:FILE_CTRL:system lib:/SourceCache/OpenSSL098/OpenSSL098-52.8.1/src/crypto/bio/bss_file.c:358:
unable to load certificate
]

Error was being passed to callback, but not handled by the callback. Just added that in my Pull Request.

[RayKahn]

Were you able to fix this issue? If so how?

Thanks