irkerd: Add InvalidRequest and use it to flatten Irker.handle()

irkerd: Add InvalidRequest and use it to flatten Irker.handle()

The old implementation had several instances of logic like this:

  if exception_condition:
      self.logerr("invalid request")
  else:
      # continue_processing

This increases nesting after each round of exception checking, and
makes the logic of the whole function harder to follow.  This commit
replaces that logic with:

  try:
      if exception_condition:
          raise InvalidRequest("invalid request")
      # continue peocessing
  except InvalidRequest, e:
      self.logerr(str(e))

Because the guts of the handle() function are already inside a
try/except block, we can add our except clause to the existing block,
and now exception checks don't increase nesting at all.

The exception to this global try/except block is the 'URL has
unexpected type' error, where we do want a local try/except block
inside the channel loop.  That way we get both errors about invalid
URLs and continue to attempt valid URLs.  This matches the existing
logic for this check, but conflicts with the current target.valid
check (which doesn't log an error and does stop processing of further
channels).