add Error() assertions on the final error value of multi-return value functions

[thediveo]

This PR follows up on #392, implementing a new Error method on Assertions.

• adds Error method to Assertion, returning a new Assertion configured for error-related assertions (see below for details).
• refactors Assertion to store both the actual value as well as all extras in a single slice for uniform handling.
• refactors extra values vetting into generic slice of actual values vetting with optionally skipping a particular element, either the first element for normal assertions and the last element for error assertions.
• an Error-returned Assertion is configured to vet differently from non-error-related assertions in order to correctly handle the Go error idiom, where either (A) all return values must be zero values in case of a trailing non-nil error, or (B) there are no constraints on the return values when the trailing error value is nil.
• changes vet-related error messages to refer to "Unexpected non-nil/non-zero argument ..." instead of "Unexpected non-nil/non-zero extra argument".
• adapts existing tests to the new vet-related error messages.
• adds new test table covering Error.

[onsi]

Looks great, thanks!