Provide sample command to install missing platform

[alranel]

As of now, when trying to compile or upload to a platform that is not installed, the following error is returned:

Error during Upload: incorrect FQBN: platform arduino:avr is not installed

It could be nice to help users by providing the command they can run to fix the issue:

Error during Upload: incorrect FQBN: platform arduino:avr is not installed; try running `arduino-cli core install arduino:avr`

(It would be even better to prompt user to install the platform, but this would break the current non-interactive behavior of arduino-cli)