Fix bugs in timestamp

[popematt]

• Fix incorrect handling of positive mantissa in fractional seconds for a zero coefficient
• Removes some panics in favor of returning Result::Err

Issue #, if available:

None

Description of changes:

While working on fixing up ion-hash for the next ion-rs release, I discovered some minor issues in the timestamp implementation. This PR fixes them.

• TimestampBuilder will now set fractional seconds to None if the fractional seconds coefficient is 0 and the exponent is > -1.
• Added test case for the above change
• Two panics are replaced by returning an EncodingError
• Added encoding_error functions that mirror the existing decoding_error functions.
• Nightly build for the coverage workflow needed to be updated because the prior nightly build was panicking for my changes (even though stable build worked fine).

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.