Improve error messages when external tool execution fails

Currently, when executing some tools via `subprocess.Popen`, the error message that shows up when the tool (e.g. npx) is not available is not very user friendly -- e.g. see crytic/slither#1103

An error message explaining that the tool could not be executed and suggesting the user to install it (or check their PATH) would be very nice to have.

Locate the subprocess.Popen call sites involved in external tool execution, including the path used for tools such as npx, and inspect how unavailable executables are handled. Done means failures clearly explain that the tool could not be executed and suggest installing it or checking PATH.

Written by the indexing model from the issue text.