improve serialization/deserialization struct error

[tkruse]

To reproduce, using two shells(e.g.):

rostopic echo /footest
rostopic pub /footest  "{x: 0.0, y: 0.0, z: 0.0.1}" 
[WARN] [WallTime: 1377942406.224566] Inbound TCP/IP connection failed: required argument is not a float

Note the topic listener is required, else no packing happens. (Sidenote: I am not sure whether this should be a warning, seems more like an error to me.)

The problem here is that the message does not contain a hint to what value was not a float. I looked at the code genpy generates for messages / service, which looks like this:

 for val1 in ...:
    _v1 = ...
    _v2 = _v1....
    _x = _v2
    buff.write(_struct...(_x....)

At least the value of _x should IMO be used in the case of errors, to indicate the writing of what failed. In my case (not using geometry_msgs/Point but a more complex message) that would have reduce me looking for the error a lot.

So a change to genpy would be to change the generated code something like this:

- except struct.error as se: self._check_types(se)
- except TypeError as te: self._check_types(te)
+ except struct.error as se: self._check_types(self._check_types(ValueError("%s: %s when writing %s" % (type(se), str(se), str(_x))))
+ except TypeError as te: self._check_types(self._check_types(ValueError("%s: %s when writing %s" % (type(te), str(te), str(_x)))))

Also see:
http://stackoverflow.com/questions/696047/re-raising-exceptions-with-a-different-type-and-message-preserving-existing-inf