Improved error handling

[jon-axon]

Error handling is currently handled fairly robustly from a pattern perspective, with the crate-specific Error enum used to differentiate multiple variants of internal error, and all Chipolata back-end functions bubbling-up any such errors to the hosting application. However at present the enum variants contain no data fields that hold details of the actual errors; all that's reported is the type of error e.g. "invalid operands" (but without saying what the operands are. This should be expanded:

• Data pertinent to the specific error should be added to the enum
• Surrounding context (e.g. CPU cycles) should also be added

[jon-axon]

This has now been addressed by:

• Renaming the previous Error enum to ErrorDetail, and adding fields to many of the variants to allow details of the specific error to be captured
• Adding a new ChipolataError struct, that holds a StateSnapshot representation of the processor at the point of error, plus an associated ErrorDetail instance
• Public methods of Processor now return ChipolataError, while lower-level functions bubble-up ErrorDetail instead (captured within the ChipolataError before Processor passes this back to the hosting application