Wrap errors in lookups with RestException. Fixes #189

[Xarthisius]

Girder throws nice errors now. They're not parsed and showed to the user in the dashboard, but 500s weren't either.

[codecov]

Codecov Report

Merging #190 into master will increase coverage by 0.37%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #190      +/-   ##
==========================================
+ Coverage   85.21%   85.58%   +0.37%     
==========================================
  Files          34       34              
  Lines        2036     2047      +11     
==========================================
+ Hits         1735     1752      +17     
+ Misses        301      295       -6

Impacted Files	Coverage Δ
server/rest/repository.py	100% <100%> (ø)	⬆️
server/lib/http_provider.py	84% <0%> (+2%)	⬆️
server/lib/resolvers.py	96.87% <0%> (+7.81%)	⬆️

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[craig-willis]

Confirmed via following test case:

• Register "doi:123/213" or "http://nonexisting.com"
• Note no traceback in Girder logs
• Note 400 error with response body containing error message in console

Now we just need Dashboard to display the error...