message: Re-introduce the RTCError move constructor/assignment
  They appear to have been unintentionally removed in a previous CL:
  Cr-Commit-Position: refs/heads/master@{#30185}
diff: @@ -115,7 +115,9 @@ class RTC_EXPORT RTCError {
  // but copy and assignment are provided for those cases that need it.
  // Note that this has extra overhead because it copies strings.
  RTCError(const RTCError& other) = default;
  + RTCError(RTCError&&) = default;
  RTCError& operator=(const RTCError& other) = default;
  + RTCError& operator=(RTCError&&) = default;
  // Identical to default constructed error.
  //
  