Cannot use plugin if user has no credential manager installed

[Grimeh]

If a user has no credential manager installed (they don't want their passwords saved, it's a shared workstation, whatever reason), connecting to source control from within the editor fails.

There should be an authorisation prompt for this situation instead of failing, seeing as this is a valid way to use Git.

Error:

Repro:

1. install Git without a credential manager
   
2. make new UE4 project with starter content
3. try to connect to source control with Git LFS 2, with "Uses Git LFS 2 File Locking workflow" tickbox ticked

[SRombauts]

It would be a nice improvement, but currently it's not withing the scope I wanted to tackled, I mentioned in the README