[RFC] Exception is VDSM

[Saggi Mizrahi]

Currently we have very specific exceptions.
This causes proliferation of exception with no real gain.

There is really no benefit for each call to throw it's own error message:
For instance, 
MiscFileReadException
MiscFileWriteException
MiscBlockReadException
MiscBlockWriteException
* There are about a 100 of these.

Could all just be "general exception". The user knows what command it ran there is no need have the exceptions specify that.

Also exception like:
ImageDoesNotExistInSD
StoragePoolMasterNotFound
StoragePoolUnknown
VMPathNotExists

Actually just mean ENOENT or EntityDoesNotExist in different contexts. There is no real reason to distinguish between them because they are also context driven.

Cutting down on the amount of exceptions to error archetypes similar to standard c errors will give us simpler API.
Also, having a specific set of exceptions per call means that every new API means a new group of supported exceptions. This makes forward compatibility very problematic.