error: function_clause, fabric_doc_update:handle_message (on view update?)

Under heavy load and insufficient CPU, a 3-node cluster produced a large number of function_clause errors. This is one of these errors.

View code and database content has been replaced by `...`
```
[error] 2018-05-28T07:14:39.496000Z couchdb@node1 emulator -------- Error in process <0.149.770> on node 'couchdb@node1' with exit value:
{function_clause,[{fabric_doc_update,handle_message,[{error,function_clause},{shard,<<"shards/20000000-3fffffff/db.1519652673">>,'couchdb@node1',<<"db">>,[536870912,1073741823],#Ref<0.0.58720260.121950>},{2,1,2,[{{shard,<<"s
hards/20000000-3fffffff/db.1519652673">>,'couchdb@node1',<<"db">>,[536870912,1073741823],#Ref<0.0.58720260.121950>},[{doc,<<"_design/Users">>,...]},{[{<<"views">>,...}]}]},{{shard,<<"shards/20000000-3fffffff/db.1519652673">>,'couchdb@node3',<<"db">>,[536870
912,1073741823],#Ref<0.0.58720260.121952>},[{doc,<<"_design/Users">>,...]},{[{<<"views">>,...}]},[],false,[{
ref,#Ref<0.0.58720260.121949>}]}]}],{dict,...]},{[{<<"views">>,{[{...}]},[],false,[{ref,#Ref<0.0.58720260.121949>}]},noreply]],[],[],[],[],[],[],[],[],[],[]}}}}],[{file,"src/fabric_doc_update.erl"},{line,55}]},{rexi_utils,process_mailbox,6,[{file,"src/rexi
_utils.erl"},{line,55}]},{fabric_doc_update,go,3,[{file,"src/fabric_doc_update.erl"},{line,39}]},{fabric,update_docs,3,[{file,"src/fabric.erl"},{line,259}]},{fabric_doc_open,read_repair,1,[{file,"src/fabric_doc_open.erl"},{line,133}]},{fabric_doc_open,go,
3,[{file,"src/fabric_doc_open.erl"},{line,53}]},{couch_index_server,'-get_index/3-fun-0-',2,[{file,"src/couch_index_server.erl"},{line,70}]}]}
```

Start at src/fabric_doc_update.erl line 55 and trace the call chain through fabric.erl and src/fabric_doc_open.erl, using the reported view-update and read-repair path as the entry point. Reproduce the function_clause under heavy load on a 3-node cluster, then confirm the error is prevented and the affected update path remains functional.

Written by the indexing model from the issue text.