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

[benbellick]

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.