dataprep.rdata should return empty rowingdata object upon failure

[Sander Roosendaal]

currently returns 0 which is not very pythonic

Should return an empty rowingdata object (need to define what that is) or raise an error.

As this routine is used frequently, need to watch out for consequences in other parts of the code.

[Sander Roosendaal]

Rowingdata 1.7.0 has empty rowingdata object