Change error returned from start functions to custom enum

[msrd0]

This helps future-proofing the API. Thanks @FSMaxB for suggesting this in #568

[codecov]

Codecov Report

Merging #571 (ed3f94c) into master (d4f3780) will decrease coverage by 0.06%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #571      +/-   ##
==========================================
- Coverage   85.26%   85.19%   -0.07%     
==========================================
  Files         111      111              
  Lines        5768     5768              
==========================================
- Hits         4918     4914       -4     
- Misses        850      854       +4     

Impacted Files	Coverage Δ
gotham/src/lib.rs	88.00% <ø> (ø)
gotham/src/tls/mod.rs	17.64% <0.00%> (ø)
gotham/src/plain/mod.rs	86.66% <100.00%> (ø)
gotham/src/middleware/session/backend/memory.rs	87.00% <0.00%> (-4.00%)	⬇️

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