Translate gRPC FailedPrecondition as HTTP PreconditionFailed

[slomek]

After gRPC documentation (https://github.com/grpc/grpc-go/blob/master/codes/codes.go#L87-L91):

// (c) Use FailedPrecondition if the client should not retry until
// the system state has been explicitly fixed. E.g., if an "rmdir"
// fails because the directory is non-empty, FailedPrecondition
// should be returned since the client should not retry unless
// they have first fixed up the directory by deleting files from it.

Returning HTTP status code 400 (BadRequest) suggets that the problem is caused by the user's input, whereas it should indicate that the system is in an invalid state.