[cpp][autogen] Show a message instead of throwing for unsupported property

[ddemidov]

This changes the C++ demos to show a message instead of throwing if a property is unsupported by specific device. Example output of the ev3dev-lang-test.cpp:

Found large motor
  Current properties are:
    Commands: [ reset run-direct run-forever run-timed run-to-abs-pos run-to-rel-pos stop ]
    Count Per Rot: 360
    Driver Name: lego-ev3-l-motor
    Duty Cycle: 0
    Duty Cycle SP: 50
    Encoder Polarity: normal
    Polarity: normal
    Port Name: outB
    Position: 267
    Position P: [not supported by the device]
    Position I: [not supported by the device]
    Position D: [not supported by the device]
    Position SP: 100
    Speed: 0
    Speed SP: 0
    Ramp Up SP: 0
    Ramp Down SP: 0
    Speed Regulation Enabled: off
    Speed Regulation P: 1000
    Speed Regulation I: 60
    Speed Regulation D: 0
    State: [ ]
    Stop Command: coast
    Stop Commands: [ brake coast hold ]
    Time SP: 0

See #53 for details

[dlech]

"Not supported" is not the only possible error here. For example, you could get -EINVAL if an argument is invalid or a number of other errors. I would think it would be better to use strerror() to get the actual error message.

[ddemidov]

Addressed in #56