feature: better error message when branch does not exist

[smjonas]

Did you check the docs?

• I have read all the lazy docs

Is your feature request related to a problem? Please describe.

I just saw this error message for one of my plugins after running :Lazy check and had to look in the code to see what it means:

Describe the solution you'd like

A better error message, e.g. branch "preview" does not exist

Describe alternatives you've considered

None

Additional context

For some reason, when running :Lazy update and the branch does not exist, the default main branch is selected. That seems like a minor inconsistency between the behaviors of check and update to me.

[smjonas]

Thank you!