CBTransfer sane error message when reading unsupported files.

cbtransfer does not support reading couchstore files from older versions. If a user uses happens to do this they will get a traceback with no indication of what the issue is.

Traceback (most recent call last):
File "_ctypes/callbacks.c", line 314, in 'calling callback function'
File "/opt/couchbase/lib/python/couchstore.py", line 408, in callback
fn(DocumentInfo._fromStruct(docInfoPtr.contents, self))
File "/opt/couchbase/lib/python/pump_sfd.py", line 218, in change_callback
cas, exp, flg, flex_meta, dtype = struct.unpack(SFD_REV_META, doc_info.revMeta)
struct.error: unpack requires a string argument of length 18
Traceback (most recent call last):^C

cbtransfer should instead report an clear error message, saying something like this maybe:

Error: Cannot read file X because it is version Y. Can only read version Z