certtool: Clarify error diagnostic message
When trying to load a private key and failing for any reason, the diagnostic message sent to the standard error stream does not make it clear that an error happened.
$ file foo
foo: cannot open `foo' (No such file or directory)
$ certtool --generate-request --load-privkey=foo --outfile=bar
Generating a PKCS #10 certificate request...
reading --load-privkey: foo
$ echo $?
1
We clarify it by prepending "error" to the message.
Signed-off-by: Thibault Nélis tn@osimis.io