enhance: add a status call for failed compilation

[ecrupper]

This will greatly ease the amount of sleuthing users have to do when they don't see a Vela status show up on their PR / commit.

Further, the context URL will point to the hooks page, which should take users right to the error message.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.

Project coverage is 52.79%. Comparing base (e0c2f7e) to head (c07d416).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
scm/github/repo.go	0.00%	5 Missing ⚠️
api/webhook/post.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1178      +/-   ##
==========================================
- Coverage   52.81%   52.79%   -0.03%     
==========================================
  Files         551      551              
  Lines       19211    19219       +8     
==========================================
  Hits        10146    10146              
- Misses       8501     8509       +8     
  Partials      564      564              

Files with missing lines	Coverage Δ
api/webhook/post.go	0.00% <0.00%> (ø)
scm/github/repo.go	75.69% <0.00%> (-0.86%)	⬇️

[KellyMerrick]

lgtm