Provide better errors messages in commandline

Instead or only printing the help message, which is not always helpful.
For example, when upgrading from prometheus v1, the retention time value
format has changed and now only accepts one unit (e.g. "15d") where it
previously allowed more complex strings (e.g. "360h0m0s").

This commit provides the error message as an explanation for the parsing
failure.

cmd/prometheus/main.go

@@ -175,6 +175,7 @@ func main() {
 
 	_, err := a.Parse(os.Args[1:])
 	if err != nil {
+		fmt.Fprintln(os.Stderr, errors.Wrapf(err, "Error parsing commandline arguments"))
 		a.Usage(os.Args[1:])
 		os.Exit(2)
 	}