Build should fail if keyName or keyPassphrase configuration elements specify invalid values

When configuring the plugin for RPM signing, if an invalid keyName or keyPassphrase is specified, the signing of the rpm is skipped. In these cases the build produces an artifact that does not meet the specifications provided in the pom ( it's not signed ). This should result in the failure of the build.

<pre>
[INFO] spawn rpm --define _gpg_name Non-Existent-Keyname --addsign rpm-signing-test-1.0.0-SNAPSHOT20120501162200.noarch.rpm
[INFO] Enter pass phrase:
[INFO] gpg: skipped "Non-Existent-Keyname": No secret key
[INFO] gpg: signing failed: No secret key
[INFO] Pass phrase check failed
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
</pre>