Add in error variables for the test failures message

When tested with the -race flag in travis it was properly catching that
these arguments to Errorf had not been set properly.

```
command/ssh_test.go:143: Errorf format %q reads arg hashicorp#1, but call has
only 0 args
```

command/ssh_test.go

@@ -140,7 +140,7 @@ func TestParseSSHCommand(t *testing.T) {
 
 			hostname, username, port, err := cmd.parseSSHCommand(test.args)
 			if err != test.err {
-				t.Errorf("got error: %q want %q")
+				t.Errorf("got error: %q want %q", err, test.err)
 			}
 			if hostname != test.hostname {
 				t.Errorf("got hostname: %q want %q", hostname, test.hostname)