Add SQ_INTERNAL_ERROR for reporting internal client errors

Summary:
When the client detects an internal invariant violation -- a state it believes
is unreachable -- it currently aborts the process with a CHECK. That takes down
everything running in the process because of a client-side bug that affects a
single operation.

Add SQ_INTERNAL_ERROR to the SquangleErrno enum so those sites can instead fail
the operation and report an error to the caller. This commit only introduces
the value; converting the individual CHECK sites to use it is separate. The
addition is purely additive -- no existing enumerator is renumbered.

Reviewed By: aditya-jalan

Differential Revision: D116688926

fbshipit-source-id: 233270d720b589c01e25e66576484221305a6d63

Author: Jay Edgar

third-party/squangle/src/squangle/mysql_client/Operation.h

@@ -107,6 +107,7 @@ enum class SquangleErrno : uint16_t {
   SQ_ERRNO_FAILED_CONFIG_INIT = 7005,
   SQ_INVALID_API_USAGE = 7006,
   SQ_INITIALIZATION_FAILED = 7007,
+  SQ_INTERNAL_ERROR = 7008,
 };
 
 // prefix to use in mysql errors generated by the client