eventerror: don't explode on missing keys while formatting

[mattrobenolt]

This helps as messages evolve over time. Previous events will be stored
without keys that are needed if the message in turn, now expects it.
This allows things to gracefully roll forward.

@getsentry/platform

[mitsuhiko]

don't subclass dict. this means we copy out the thing for no good reason. Alternatively proposal:

class X(object):
 def __init__(self, x):
  self.x = x
 def __getitem__(self, x):
  try:
   return self.x[x]
  except LookupError:
   return ''

[mattrobenolt]

Done.