app error handling using anyhow

[extrawurst]

now that we have nice error coverage in asyncgit we could take the next step and add error support to the main application using anyhow based on the great summary on error handling in rust:

my idea would be to show an error popup similar to this one where ever possible: #74 (comment)

if we come across unrecoverable errors we might need to look into a second path aswell which will close the application.

[extrawurst]

one problem is going to be external libraries (dependencies) that do not go full no-unwrap too. Currently we have one of those issue with tui-rs: #72

[MCord]

I am working on this.