Use gRPC error codes instead of fmt.Errorf

[merenbach]

Closes #512

Output with this patch when adding, for instance, an app at an invalid path, yielding an embedded FailedPrecondition error code:

$ ./dist/argocd app create --name guestbook-default --repo https://github.com/argoproj/argocd-example-apps.git --path guestbook2 --env default --dest-server https://`minikube ip`:8443
FATA[0004] rpc error: code = InvalidArgument desc = application spec is invalid: InvalidSpecError: Destination server and/or namespace missing from app spec;InvalidSpecError: Unable to generate manifests in guestbook2: rpc error: code = FailedPrecondition desc = Failed to read dir /var/folders/cf/yq01t3c11s51x5fqkn8065p1jvtw07/T/https:__github.com_argoproj_argocd-example-apps.git/guestbook2: open /var/folders/cf/yq01t3c11s51x5fqkn8065p1jvtw07/T/https:__github.com_argoproj_argocd-example-apps.git/guestbook2: no such file or directory 

Before the patch, the embedded error has a code of Unknown:

$ ./dist/argocd app create --name guestbook-default --repo https://github.com/argoproj/argocd-example-apps.git --path guestbook2 --env default --dest-server https://`minikube ip`:8443
FATA[0006] rpc error: code = InvalidArgument desc = application spec is invalid: InvalidSpecError: Destination server and/or namespace missing from app spec;InvalidSpecError: Unable to generate manifests in guestbook2: rpc error: code = Unknown desc = Failed to read dir /var/folders/cf/yq01t3c11s51x5fqkn8065p1jvtw07/T/https:__github.com_argoproj_argocd-example-apps.git/guestbook2: open /var/folders/cf/yq01t3c11s51x5fqkn8065p1jvtw07/T/https:__github.com_argoproj_argocd-example-apps.git/guestbook2: no such file or directory