Improve handling of unknown commands in CInv::ToString

CInv::GetCommand throws an exception if the command type is unknown. Consequently, CInv::ToString fails with an exception if the command type is unknown. 1.5 introduces such a new command type with filtered blocks. A ToString method throwing an exception on a valid object encoding unknown data does not look like expected behaviour and is the likely cause of #65.

Since I can't find any mention of a release of 1.4.3 I haven't incremented the version number. This fix applies equally to the current version and depending on the feedback here I will open a matching PR for the develop-branch.