improve error message when unable to get an input file

In this case, the compute program is run the same as if addcomputed --fast
were used, so it should succeed, without outputting a computed file.

computeInputsUnavailable is in ComputeState for simplicity, but it is
not serialized with the rest of the ComputeState.

Command/AddComputed.hs

@@ -95,6 +95,7 @@ perform o r = do
 		, Remote.Compute.computeOutputs = mempty
 		, Remote.Compute.computeSubdir = subdir
 		, Remote.Compute.computeReproducible = False
+		, Remote.Compute.computeInputsUnavailable = False
 		}
 	fast <- Annex.getRead Annex.fast
 	Remote.Compute.runComputeProgram program state