Improve error reporting for dependencies

[cynthi8]

• I'm submitting a ...

  • feature request

• What is the current status?
  Any sort of failure in the connection process will result in a "No device connected" message. Often times this is due to the python3 binary not being found due to incorrect executable name (Python 3 may not be found though installed #655) or missing environmental variables (App doesn't connect to device on Mac #684).

This can be confusing for users as the real issue is with the dependencies and not the device connection.

• What is the expected behavior?
  If the python shell cannot be started, or the pslab library cannot be found, this should be reported directly to the user.

• Would you like to work on it?
  Yes.