Improve UX and error handling when pushing libraries

[michaelrambeau]

While working with @Evomatic on an issue about branch names that include a slash, we noticed several points to improve.

There is something wrong about the error handling, at some point I got a message ERROR: followed by nothing, impossible to figure out what was wrong

At some point I got a weird error in the terminal, it looks like a HTML error page displayed character by character, this kind of error should be handled in a better way.

Pushing takes a lot of time sometimes while nothing is on the screen: a spinner could help (related #38 )

And last thing, when everything is fine, the final success message in blue includes an invalid character (I guess it was an emoji) that gets replaced by either a question mark or a square, depending on the terminal.