create a common error type with unwrap capability

[chappjc]

We should have a dex error type that works well with the Go 1.13 error handling functions, as summarized in decred/dcrd#2181

The described approach is what I did for the error type used in the server/db/drivers/pg package that implements Unwrap. This could be the basis for such a type, dex.DexError perhaps:

dcrdex/server/db/driver/pg/errors.go

Lines 14 to 38 in ecbeacc

	// DetailedError pairs an Error with details.
	type DetailedError struct {
	wrapped error
	detail string
	}
	// Error satisfies the error interface, combining the wrapped error message with
	// the details.
	func (e DetailedError) Error() string {
	return e.wrapped.Error() + ": " + e.detail
	}
	// Unwrap returns the wrapped error, allowing errors.Is and errors.As to work.
	func (e DetailedError) Unwrap() error {
	return e.wrapped
	}
	// NewDetailedError wraps the provided Error with details in a DetailedError,
	// facilitating the use of errors.Is and errors.As via errors.Unwrap.
	func NewDetailedError(err error, detail string) DetailedError {
	return DetailedError{
	wrapped: err,
	detail: detail,
	}
	}

Usage example in the unit test:

dcrdex/server/db/driver/pg/errors_test.go

Lines 8 to 20 in ecbeacc

	func TestDetailedError(t *testing.T) {
	// Ensure that DetailedError.Unwrap is working via errors.Is.
	detail := "blah"
	detailed := NewDetailedError(errTooManyRows, detail)
	if !errors.Is(detailed, errTooManyRows) {
	t.Errorf("Failed to recognize this NewDetailedError as errTooManyRows.")
	}
	expectedErr := errTooManyRows.Error() + ": " + detail
	if detailed.Error() != expectedErr {
	t.Errorf("Wrong error message. Got %s, expected %s", detailed.Error(), expectedErr)
	}
	}

Originally raised by @JoeGruffins in #477 (comment)

[chappjc]

Resolved by #484, but it still needs to be put into wider use.