Improve error messages when external tool execution fails

[elopez]

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.