Improvement : Error when TTS engine is missing (none) is not explicit #140

Hello,

thank you for this amazing app.

I realised that after I disable TTS Google Engine.

Here below, when I trigger a timer and none TTS engine exists, no speech is played, instead it is displayed : "Text to speech error : -1"

I managed to guess what was going on. But, to improve a bit the flow, could this error be a bit more detailed / explicit, please ?

For example : "No Text To Speech engine is defined".