Inform users `utop` is missing on M-x utop

If `utop` is not installed and the user invokes `M-x utop`, all that occurs is the following vague error message in a new buffer:
```
Process utop exited with code 1
```

I propose that if utop is not found, just present an error in the mini buffer. 

Also to be clear, I will take this one on, but just wanted to make the problem known and make it possible to discuss.