PhysicalFilesWatcher should defend against PathTooLongException

[Eilon]

In the various event handlers in PhysicalFilesWatcher, there can be exceptions thrown for paths that are too long. For example, constructing a FileInfo with a long path can throw.

Because this code runs in background threads, this can easily take down a process that's using it.

@NTaylorMullen did some related fixes in #187, but it seems that we need to handle more cases.

See Watson info here: https://devdiv.visualstudio.com/DefaultCollection/DevDiv/_workitems?id=186004&_a=edit

[BrennanConroy]

b58bbe0