Fix docs generator when git executable is missing

[straight-shoota]

The code for executing the git program didn't take into account that Process.run raises if the executable can't be found. This is now fixed. I also refactored the code for capturing the program output (DRY).

Fixes #9789

[bcardiff]

Should we do the same on crystal init later?

[straight-shoota]

I noticed there are similar cases, but I just focused on the more common problem here. I'll make another PR for init.

[straight-shoota]

Given that it's quite common to ignore failing programs (like status = Process.run("command"); return unless status.success?), maybe we should consider a variant Process.run? that doesn't raise on exec error. It could just return nil which is still easier to handle than to rescue IO::Error. Alternatively, perhaps Process::Status could be extended for this use case, so that the above example code would (with Process.run?) just work even if the command can't be executed.

[bcardiff]

Closing in favor of #9893