message: Remove one more crypto::tink::util::error::UNKNOWN.
  This CL is a part of Tink migration to using absl::Status{,Or} and absl::StatusCode.
diff: @@ -45,7 +45,7 @@ namespace tink {
  overload(__VA_ARGS__); /* Ignoring return value. */ \
  return util::Status(); \
  } catch (const std::exception &e) { \
  - return util::Status(util::error::UNKNOWN, e.what()); \
  + return util::Status(absl::StatusCode::kUnknown, e.what()); \
  } catch (...) { \
  std::abort(); \
  }
  @@ -66,7 +66,7 @@ namespace tink {
  auto o = overload(__VA_ARGS__); \
  return o.cast<statusor_payload_type>(); \
  } catch (const std::exception &e) { \
  - return util::Status(util::error::UNKNOWN, e.what()); \
  + return util::Status(absl::StatusCode::kUnknown, e.what()); \
  } catch (...) { \
  std::abort(); \
  }
  