Return error when creating gitlabdownloader failed

[lunny]

related with #12779

[codecov-commenter]

Codecov Report

Merging #12790 into master will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master   #12790      +/-   ##
==========================================
+ Coverage   43.23%   43.24%   +0.01%     
==========================================
  Files         650      650              
  Lines       72048    72041       -7     
==========================================
+ Hits        31150    31155       +5     
+ Misses      35852    35842      -10     
+ Partials     5046     5044       -2     

Impacted Files	Coverage Δ
modules/migrations/gitlab.go	1.06% <0.00%> (+0.01%)	⬆️
modules/indexer/stats/queue.go	64.70% <0.00%> (-11.77%)	⬇️
modules/indexer/stats/db.go	43.47% <0.00%> (-8.70%)	⬇️
modules/charset/charset.go	68.53% <0.00%> (-4.50%)	⬇️
modules/git/utils.go	73.77% <0.00%> (-3.28%)	⬇️
modules/queue/workerpool.go	58.77% <0.00%> (-1.23%)	⬇️
models/gpg_key.go	54.81% <0.00%> (-0.59%)	⬇️
modules/git/repo.go	49.23% <0.00%> (-0.51%)	⬇️
models/error.go	35.66% <0.00%> (+0.85%)	⬆️
models/notification.go	67.04% <0.00%> (+0.91%)	⬆️
... and 5 more

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[6543]

thanks