Made minor refactoring to handle exceptions from 'Source' and 'Destination'
in a consolidated manner

1) SourceExceptions are thrown when installing a package locally
from Source fails
2) DestinationExceptions are thrown when pushing a package
to the destination fails
Merge pull request #36 from NuGet/daravind/NuGetV2RepositoryMirrorWork

Made minor refactoring to handle more exceptions