Make the joint encoder diagnostic more general as possible

[valegagge]

A long time ago the only joint encoder available were the AEA and in the diagnostic message we refer to them. Now we have different type of joint encoder, so we need to make the diagnostic message more general.

For example in this issue robotology/icub-tech-support#1901, the user was left blindsided.

Dod
The joint encoder diagnostic messages are clearer.