Blockly available even if Python3 not installed

[sgallou]

We can create a Blockly rule even if Python3 is not available on the system. Rule will crash at run start. We must have the same behavior as for Python3 interpreter, which doesn't permit to create a rule if Python3 not available on the system.

Steps to reproduce : On a system (tested with Pi0w) without Python3 installed :

• Create a Blockly rule, add some blocks and validate ==> Error with a generic message
• Try to create a Python3 rule : can not enter editor as Python3 not installed, and a understandable message is displayed

[sgallou]

Closed by 2ba8a26e1acd4b5da21b7bbb8e104ecf13b7094c