Standardize error reporting

[kdubb]

Currently thrown errors are very "ad hoc". Shield needs to standardize error reporting and structures.

• Common Protocol (e.g. ShieldError) that all error enums/structs implement.
• Universally support NSUnderlyingErrorKey and/or NSMultipleUnderlyingErrorsKey
• OSStatus check failures need to include the status as an underlying error.
• CCStatus/CCCryptorStatus check failures need to include the status as an underlying error.