In the event that we cannot load our plugins, we shouldn't raise a
cryptic KeyError from our formatter.

Closes #320

Closes #205

Serialize Checkers PluginTypeManager to a dict

*Description of changes*

Try to side-step issues with attributes not being set/updated on plugins when used with multiprocessing and Queues.

*Related to:*  #179, #164 

See merge request !98

...and avoid .strerror altogether since py3 does not have it.
See #203.

It seems likely that the multiprocessing module on Windows is not
capable of serializing an object with the structure that we have and
preserving the attributes we dynamically set on plugins (like the
FlakesChecker). To avoid issues like this with all plugins (although
we have only found this on Windows with the FlakesChecker), let's try
serializing the Checkers PluginTypeManager to a dictionary so that the
only object that a Queue is really trying to serialize/deserialize is
the FlakesChecker itself.

Related to #179

Plugins do not have their parameters checked in advance, so when we
try to aggregate parameters for a plugin, there's a chance it may
request an attribute of the FileProcessor that simply does not exist.

We catch this and re-raise it but we should also capture it in the
checker manager and handle it appropriately there as well.

This fixes a regression in behaviour from 2.x to 3.

Closes #150

This is an emerging best practice and there is little reason to not
follow it