Fix download RPC

[rkervella]

Return an error if the file doesn't exist.

[RafBishopFox]

If we need a specific error returned for the condition where a file does not exist, then no problems. If the operator requests a filter, like C:\*.txt, and no files exist, this error will not be triggered. If we need that, then we will have to introduce that logic.

[RafBishopFox]

Upon further review, this breaks filtering because the a request like C:\*.txt will throw that error.

[RafBishopFox]

Maybe we need to have logic to detect if a filter that is not a specific filename is involved, and if so, then let os.IsNotExist pass.

[RafBishopFox]

Never mind. Operator error. Filtering seems fine.