Fix Distinct in MongoStore

[shyamd]

Fix the distinct operation from failing in the MongoStore when too much data is being pulled. This broke because pymongo made an explicit DocumentTooLarge error that was not being caught.

[codecov]

Codecov Report

Merging #332 (0d54956) into master (6661534) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #332      +/-   ##
==========================================
+ Coverage   87.41%   87.51%   +0.09%     
==========================================
  Files          28       28              
  Lines        2170     2170              
==========================================
+ Hits         1897     1899       +2     
+ Misses        273      271       -2     

Impacted Files	Coverage Δ
src/maggma/stores/mongolike.py	82.42% <100.00%> (+0.83%)	⬆️

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