Provide a clearer error message when the `stree` exe cannot be spawned

[xeger]

STR:

1. Install this extension, but without installing the gem. (And/or without having a ruby interpreter in your path.)
2. Open a Ruby source file

Outcome: useful but slightly cryptic error messages.

These are fine, but could possibly be improved.

Desired:

• Human-readable error message & maybe some actionable advice ("install the gem" "install a Ruby")
• Maybe we could even prompt the user to install the system gem for them, as the Solargraph extension does. This is getting awfully fancy however.

[kddnewton]

@xeger this is good to close now right?

[xeger]

Yup, go for it.