Properly log tracebacks

[meejah]

The first argument to log.critical() is actually the format-string -- so if a traceback itself contains {}'s then this becomes a "cannot format log message" error.

[codecov-io]

Current coverage is 57.34%

Merging #613 into master will not affect coverage as of 199d61f

@@            master    #613   diff @@
======================================
  Files           61      61       
  Stmts        10238   10238       
  Branches      1664    1664       
  Methods          0       0       
======================================
  Hit           5871    5871       
  Partial        556     556       
  Missed        3811    3811       

Review entire Coverage Diff as of 199d61f

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