Improve error message when mono is not installed

[markfinal]

It would be nice to be able to say something better than this:

mark@ubuntu16:~/Downloads/BuildAMation-1.0.3$ source env.sh 
/home/mark/Downloads/BuildAMation-1.0.3/bin/Release/bam: line 44: exec: mono: not found

Perhaps some references to where to download it?

[markfinal]

which mono >> /dev/null || { echo "Mono not installed"; exit 1; }

possibly?