Allow for 'git push' retry + a bit of documentation, fixes #385

[mauritsvanrees]

I released this in 9.0.0a3.

But here it erroneously thinks that something is wrong, when in fact the push goes fine:

OK to push commits to the server? (Y/n)? 
To github.com:zestsoftware/zest.releaser.git
5b7e2e1..fc443d8  master -> master
There were errors or warnings.
Perhaps the main branch is protected?
Retry this command? [Yes/no/quit/?]
INFO     Retrying command: 'git push'                                                                                                                                                                                                       
INFO     Retrying command: 'git push'                                                                                                                                                                                                       
Everything up-to-date
There were errors or warnings.
Retry this command? [Yes/no/quit/?]n
INFO     Everything up-to-date                                                                                                                                                                                                          
To github.com:zestsoftware/zest.releaser.git
* [new tag]         9.0.0a3 -> 9.0.0a3
There were errors or warnings.
Perhaps the main branch is protected?
Retry this command? [Yes/no/quit/?]n
INFO     To github.com:zestsoftware/zest.releaser.git                                                                                                                                                                                   
         * [new tag]         9.0.0a3 -> 9.0.0a3                                                                                                                                                                                         
INFO     Finished full release. 

The info returned by GitHub is interpreted as error or warning, presumably because it is printed to /dev/stderr even though they are not errors. Let me add a screen shot with colours for clarity:

For clarity, what I do myself, is:

• I let the command do a git push.
• This shows an "error".
• I retry.
• git push says that everything is up-to-date.
• I do not retry.
• Then git push tags, showing an "error"
• I do not retry.
• The commits and new tag are on GitHub just fine.