Added more negative tests to ot_test.opentxs
Fixed the following bugs while doing that:
- OTAPI_Exec::GetNym_MailCount nor returning failure for unknown nym
- 'transfer' crashing for unknown myacct
- 'sendmsg' not checking for unknown server, mynym, or hisnym
  Inserted this test BEFORE asking the user for message data
- 'sendcash' crashing for unknown hisnym
  Inserted this test BEFORE trying cash withdrawal
  Also added missing check on OTAPI return values
- 'withdraw' crashing for unknown myacct
- 'mail' not failing for unknowm mynym or invalid index
- 'outmail' not failing for unknowm mynym or invalid index
- Better display message in VerifyExists
- Don't automatically set myacct when mypurse was specified
- Don't automatically set hisacct when hispurse was specified