Fix Coerce validator to catch decimal.InvalidOperation

[geerk]

For some reason Python std lib has inconsistency in handling invalid values for numeric types:

>>> float('abc')
ValueError: could not convert string to float: abc
>>> int('abc')
ValueError: invalid literal for int() with base 10: 'abc'
>>> Fraction('abc')
ValueError: Invalid literal for Fraction: 'abc'
>>> Decimal('abc')
decimal.InvalidOperation: Invalid literal for Decimal: 'abc'

So this PR is to add InvalidOperation exception for Decimal type when using Coerce validator.

[coveralls]

Coverage remained the same at 95.309% when pulling a30155c on geerk:master into 85b96b3 on alecthomas:master.

[geerk]

@alecthomas Thanks!