Improve error messages then a VCS operation fails.

[tomwilkie]

Instead of errors like this:

solve error: No valid source could be created:
	failed to set up "", error Unable to retrieve local repo information	failed to set up "", error Unable to update repository	failed to set up "", error Unable to update repository	failed to set up "", error Unable to update repository

This changes gives us errors like this:

solve error: No valid source could be created:
	failed to set up "https://github.com/jmespath/go-jmespath", error Unable to retrieve local repo information	failed to set up "ssh://[email protected]/jmespath/go-jmespath", error Unable to update repository	failed to set up "git://github.com/jmespath/go-jmespath", error Unable to update repository	failed to set up "http://github.com/jmespath/go-jmespath", error Unable to update repository

Part of golang/dep#320

[codecov]

Codecov Report

Merging #193 into master will increase coverage by 0.34%.
The diff coverage is 12.5%.

@@            Coverage Diff             @@
##           master     #193      +/-   ##
==========================================
+ Coverage   79.15%   79.49%   +0.34%     
==========================================
  Files          24       24              
  Lines        3804     3804              
==========================================
+ Hits         3011     3024      +13     
+ Misses        596      585      -11     
+ Partials      197      195       -2

Impacted Files	Coverage Δ
maybe_source.go	80% <12.5%> (ø)	✅
analysis.go	89.87% <0%> (+3.37%)	✅

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[sdboyer]

hah, we cross-posted :)

I'll take this, and then make a couple small additions to fix my other notes in #192. thanks!

[tomwilkie]

Thanks!