Trap exception where unitprice is >int64 and drop

Since Masterchest is in maintenance mode now, this fringe case where for
example 0.00000001 tokens are listed for 100 million BTC for example,
providing for unit prices in the 1.e+23 range will be trapped and
dropped.
Merge pull request #72 from zathras-crypto/master

Trap exception where unitprice is >int64 and drop