Improve error messaging around missing executables

[msrose]

For now we assume that any executables required by adapters (jest, npm, yarn, maven, gradle, pytest, etc.) are available in the environment -- if they're not, the behaviour is currently undefined, but right now it will basically bubble up the error message from spawnSync to the user.

It would be nice to provide better error messages to the user to ease any debugging they have to do around tools that aren't installed.

Each adapter should also provide documentation as to what it expects to be present in the execution context when tests are being executed.