Improve error messaging around missing executables

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.

Start by locating each adapter's executable invocation and the current spawnSync error handling. Review the adapters for jest, npm, yarn, maven, gradle, and pytest, then document their execution-context requirements. Done means missing tools produce useful user-facing errors and each adapter explains the executables it expects.

Written by the indexing model from the issue text.