meaningful error messages (close #55)

Here's what the output should look like:

$ python3 generate.py -b ./radix-protocol -m radix/modem -l cppp -o out/cpp
ERROR: Unsupported language "cppp"
$ python3 generate.py -b ./radix-protocol -m 2radix/modem -l cpp -o out/cpp
ERROR: Path not found: ./radix-protocol/2radix/modem
$ python3 generate.py -b ./radix-protocol -m radix/modem -l cpp -o out/cpp 
Successfully generated to out/cpp

microavia / messgen

meaningful error messages (close #55) #56