Sync: Improved error message when manifest server RPC call fails

When the RPC call fails, the error message returned by the server
is printed, but it is not obvious that this is caused by RPC call
failure.

Prefix the error message with a descriptive message that explains
what went wrong.

Change-Id: I4b77af22aacc2e9843c4df9d06bf54e41d9692ff
1 file changed