WriteConflictException was not being handled properly by the write command update executor. This caused them to erroneously appear to succeed.

Original Description
The update_mms tests suddenly stopped scaling at all, where last week they did. This is something in the mongo tree, not the WT library as I keep that constant during my testing here and see very different results.

I spent a bit of time today narrowing it down and the issue happens between changeset c84d5ea and changeset 7f9e3f3 (FYI, c84d5ea is a parent of 7f9e3f3). The diff between these two changesets is very small and attached.

Here is the page of commits from GitHub:
https://github.com/wiredtiger/mongo/commits/master?page=9

Here are the results. I only show Update.MmsIncShallow1 although all show similar patterns.

GOOD:
db version: 2.7.9-pre-
c84d5ea9f484948ba4f52bf00274bd84e45b837e

Update.MmsIncShallow1
1       10495.389587181788
1       10455.777272892861
2       15549.864735174962
2       15670.030276924275
4       25134.398199163585
4       25159.574244737894
8       32214.23890536317
8       32237.931283570488
Finished Testing.

BAD:
db version: 2.7.9-pre-
7f9e3f383b0bd77e5119a5d57883631950c1a6ff

Update.MmsIncShallow1
1       10431.11424139814
1       10637.16293351225
2       11245.63626083444
2       11264.545902335614
4       10201.82719233331
4       11287.419799605117
8       12088.974243081819
8       12127.817603989664
Finished Testing.