fix: update commit status on merge error

[samcontesse]

also mark it as pending as soon as the get is triggered and failed when other errors occur.

closes #5

[SuperTux88]

So #5 is still a problem, but this was never merged, and has conflicts now. Is there a reason why this was never merged?

I thought that a workaround for #5 is to set skip_not_mergeable to true, so the merge should never fail. But that has other side-effects, as sometimes the mergeStatus is set to "unchecked" instead of "can_be_merged", because gitlab doesn't know yet if it's mergeable or not, so these MRs then sometimes end up never being built.

[samcontesse]

Closing since this repository is going to be archived.