Refactor Error Translation

[daniel-j-h]

The serial sub-system provides it's own error types to be fully encapsulated and to not go near circular dependencies. The translation to sweep's error types happens manually all over the place in sweep.c (e.g.

sweep-sdk/libsweep/sweep.c

Lines 69 to 73 in f7fbfbd

	if (serialerror) {
	*error = sweep_error_construct("unable to write command");
	sweep_serial_error_destruct(serialerror);
	return;
	}

where the serial error message is even discarded).

Refactor this translation into a helper function.

[daniel-j-h]

Update: we're using C++11 in our implementation now and only expose error codes to the outside (C API boundary). When working on this think about if it would make sense to throw sub-system exceptions catching them at the boundary. See discussion in #33 (comment).