feat: GetSignaturesProtectedBranch returns ErrBranchNotProtected when branch is not protected

[Not-Dhananjay-Mishra]

Fixes : #2821

Added this check, similar to what we already have in GetBranchProtection

if isBranchNotProtected(err) {
	err = ErrBranchNotProtected
}

Now, when a branch is not protected, GetSignaturesProtectedBranch also returns ErrBranchNotProtected

[alexandear]

Please add a unit test.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.44%. Comparing base (5dc0050) to head (e0b4cc6).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3896   +/-   ##
=======================================
  Coverage   92.44%   92.44%           
=======================================
  Files         203      203           
  Lines       14925    14927    +2     
=======================================
+ Hits        13797    13799    +2     
  Misses        926      926           
  Partials      202      202           

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[gmlewis]

Thank you, @Not-Dhananjay-Mishra and @alexandear !
LGTM
Merging.