Better error on missing schema

[palfrey]

Improves the "missing schema" error, which would improve the situation in #186

[codecov]

Codecov Report

Merging #196 (f55ebcf) into master (1185468) will increase coverage by 0.42%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #196      +/-   ##
==========================================
+ Coverage   89.85%   90.27%   +0.42%     
==========================================
  Files           1        1              
  Lines          69       72       +3     
  Branches       15       17       +2     
==========================================
+ Hits           62       65       +3     
  Misses          3        3              
  Partials        4        4              

Impacted Files	Coverage Δ
dj_database_url.py	90.27% <100.00%> (+0.42%)	⬆️

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[rtpg]

This looks good, but I would leave out the list of supported protocols that are supported. I don't think that's a blocker though.

[palfrey]

This looks good, but I would leave out the list of supported protocols that are supported. I don't think that's a blocker though.

Without that, fixing things like #186 would need digging through the source code

[rtpg]

Or, like, the documentation... but I get what you're saying. I don't feel strongly about this!