ParseHashV validates the length and encoding of the string and throws
an informative RPC error on failure, which is as good or better than
these alternative calls.
Note I switched ParseHashV to check string length first, because
IsHex tests that the length is even, and an error like:
"must be of length 64 (not 63, for X)" is much more informative than
"must be hexadecimal string (not X)"
Future commits will deprecate the accounts RPC methods, arguments and
return objects. Set the -deprecatedrpc=accounts switch now so tests
don't break in intermediate commits.
