Missing jar file fails silently

[bitcoin-coder-bob]

When calling NewServer if the jar file is not found (bad path or just does not exist), NewServer does not return an error. When the Start function is then called, it will also not return an error.

The underlying line of code being called in Start (line 77) is:
cmd := command("java", "-jar", s.jar, "-p", s.port) which is then started with:
if err := cmd.Start(); err != nil { return err } which has no problem except for the fact that it is erring silently. If we read from standard error we can see the problem being logged, but the current code is not capturing this, since cmd.Start() did not err. Thus, this is a silent "failure". The code does not fail but the call to start the Server never actually succeeds.

Consider the following example code:

server, err = tika.NewServer("tika-server-1.19.jar", "")
	if err != nil {
		l.Error().Msgf("Error creating Tika server: %s", err.Error())
		return
	} else {
		l.Info().Msgf("SERVER: %v", server)
	}
	err = server.Start(context.Background())
	if err != nil {
		l.Error().Msgf("Error starting Tika server: %s", err.Error())
		return
	} else {
		l.Info().Msg("No issue starting server")
	}
	l.Info().Msg("HERE")

This prints: SERVER: &{tika-server-1.19.jar http://localhost:9998 9998 <nil>} and HERE is never printed since the line is never reached. The Start function never completes.

I propose the following fix to be placed in the first line of the Start function:

if _, err := os.Stat(s.jar); os.IsNotExist(err) {
		return err
	}

This will allow for immediate failure if the tika jar file was not found or does not exist.

I would like to make a PR for this.