Better error messages

[piegamesde]

If you search for "An unknown error occurred", you'll find a lot of issues with a lot of different causes. This is making things harder for everybody. Please change this to a more concise error message that gives more information for the most common failure cases:

• Native extension host is not installed
• Native extension host binary cannot be found
• …

[samhh]

I think there's better handling of error messages in the PureScript rewrite but I honestly can't remember. I'll keep this open because it's certainly something to improve.