Add warnings for signing runlog commits

[jo-basevi]

Re-attempt to sign commits without signing with a warning, when signing runlog commits fails due to git config using:

• signing key with a passphrase or the private key is hosted on local machine
• or the gpg.ssh.program is not accessible on run pbs job - path is not added to storage flags

Closes #449

I thought about adding a help message, something like

If signed runlog commits are required, either:
 - manually run 'git commit --amend' to resign the latest commit
 - change git config to a  use passphrase-less and local signing key pair,
    and check configured 'gpg.ssh.program' is accessible during PBS run job (check storage flags) 

Which I can add in if it'll be helpful - I just don't want to make it even more confusing..

[coveralls]

coverage: 52.555% (+0.1%) from 52.419% when pulling 55f594a860705fb6fbd62ac80f1ec921f4914624 on ACCESS-NRI:449-signing-git-commits into e611bcca30a95be96903821d8a083baf2c8cdde7 on payu-org:master.