catch bad data types in calculations

[getodk-bot]

Originally reported by: Yaw Anokwa (Bitbucket: yanokwa, GitHub: yanokwa)

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http://code.google.com/p/opendatakit/issues/detail?id=356

if you add a calculate="200 div 100" to an geopoint (or a integer) node, there will be class cast exception at some point because that calculation isn't valid. ideally these kinds of problems should be caught by the parser.

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• Bitbucket: https://bitbucket.org/javarosa/javarosa/issue/56