Fix AksIM2 diagnostic display

[AntonioViscomi]

Hello everyone,
With this PR I would like to merge a modified version of EOappEncodersReader.c, specifically in the lines reported here.
This modification is done to avoid sending multiple error messages (invalid data, invalid crc, close to limits warning) when the encoder is retrieved as disconnected.
The test was performed by simply removing the AksIM2 connector from the EMS board and looking at the yarprobotinterface output.

The code shows the following output:

[ERROR] from BOARD 10.0.1.1 (three-encoders-setup) time=267s 275m 291u :  MC: AEA encoder invalid data. Hardware problem in the magnetic position sensor of the joint caused invalid position readings. (Joint=three_encoders_joint (NIB=0), encoderPort=0)
[ERROR] from BOARD 10.0.1.1 (three-encoders-setup) time=272s 276m 291u :  MC: AEA encoder invalid data. Hardware problem in the magnetic position sensor of the joint caused invalid position readings. (Joint=three_encoders_joint (NIB=0), encoderPort=0)
[ERROR] from BOARD 10.0.1.1 (three-encoders-setup) time=275s 746m 214u :  HW - encoder: not connected (Joint=three_encoders_joint (NIB=0) Number of error in 10 seconds is: 10001)
[ERROR] from BOARD 10.0.1.1 (three-encoders-setup) time=277s 277m 291u :  MC: AEA encoder invalid data. Hardware problem in the magnetic position sensor of the joint caused invalid position readings. (Joint=three_encoders_joint (NIB=0), encoderPort=0)
[DEBUG] yarprobotinterface running happily
[ERROR] from BOARD 10.0.1.1 (three-encoders-setup) time=282s 278m 291u :  MC: AEA encoder invalid data. Hardware problem in the magnetic position sensor of the joint caused invalid position readings. (Joint=three_encoders_joint (NIB=0), encoderPort=0)
[ERROR] from BOARD 10.0.1.1 (three-encoders-setup) time=285s 747m 214u :  HW - encoder: not connected (Joint=three_encoders_joint (NIB=0) Number of error in 10 seconds is: 10001)
[ERROR] from BOARD 10.0.1.1 (three-encoders-setup) time=287s 279m 291u :  MC: AEA encoder invalid data. Hardware problem in the magnetic position sensor of the joint caused invalid position readings. (Joint=three_encoders_joint (NIB=0), encoderPort=0)
[ERROR] from BOARD 10.0.1.1 (three-encoders-setup) time=292s 280m 291u :  MC: AEA encoder invalid data. Hardware problem in the magnetic position sensor of the joint caused invalid position readings. (Joint=three_encoders_joint (NIB=0), encoderPort=0)

Only one part of the output is reported as the `yarprobotinterface' shows this repetitive pattern.
Once the encoder is reconnected, the errors are handled as before. For more information, see the linked issue here.

[valegagge]

Just a note: this PR contains a very small change that affects only the diagnostic of Aksim, so it is not necessary to create the relative icub-fw-build PR with the new exe for all boards.