Review error handling within Decimal

[paupino]

There are some areas where panic is warranted however there are others (e.g. rescale, from_bytes_le) that we could utilize Rust's error handling further. In addition, with libraries such as error-chain we can perhaps make the error handling experience richer with little overhead.

[jekirl]

I would propose that panics be only for truly unrecoverable errors, which in this case should be limited to debug mode perhaps? In general I think there should be no panics for the Mul/Add/Div/Sub/etc traits, and instead there be a checked_mul/div/add/sub/etc function for each implemented under the Decimal trait that returns an error type (through error-chain). Perhaps having panic on release and/or debug can be a feature flag?

[paupino]

Have expanded checked operators into #80. I do like the idea of debug mode only panic's however would like to perhaps come up with a clear set of rules. For div, for example, I'm thinking:

• Div by zero = panic. If not panic, what would make sense to return? We could introduce the concept of Infinity within flags?
• Overflow = Saturate
• Underflow = Round

Thoughts?