More uniform and comprehensive exception handling

[abingham]

trafficstars

ycmd returns exceptions when it runs into problem, and we currently handle them in an ad hoc and spotty manner. We should first find all of the places where we might be getting these exceptions. We should then see if we can devise a unified way to handle them. From what I can tell, they all have essentially the same structure, so a common handler should be relatively straightforward.

[abingham]

26c6732b7ec28f2be39ca74244cbf70f49fa44c8 is a step in the right direction.