improve exception handling in run_psi4

[eendebakpt]

When Psi4 is not installed an exception occurs because the file psi4 cannot be found. Cleanup of the process with process.kill() then generates another exception, because the variable process was not created.

This PR:

• Adds the generated exception to the warning message
• Wraps the process.kill() in a try-except

@bsenjean