make the crypto error type a native go type (#8267)

This commit makes the `crypto.Error` type a native go (string)
type. That allows us to define error values as constants instead
of variables.

For reference see:
 - https://twitter.com/_aead_/status/1118170258215514115?s=20
 - https://dave.cheney.net/2016/04/07/constant-errors

cmd/api-errors.go

@@ -1723,6 +1723,12 @@ func toAPIError(ctx context.Context, err error) APIError {
 		// their internal error types. This code is only
 		// useful with gateway implementations.
 		switch e := err.(type) {
+		case crypto.Error:
+			apiErr = APIError{
+				Code:           "XKMSInternalError",
+				Description:    e.Error(),
+				HTTPStatusCode: http.StatusBadRequest,
+			}
 		case minio.ErrorResponse:
 			apiErr = APIError{
 				Code:           e.Code,