[analysis_server] Fix stacktrace handling when reporting exceptions.

Changes the null check in `if (stackTrace != null && exception is!
CaughtException)` from `!=` to `==`. The test was clearly intended to
supply a stack trace if (a) there isn't one already, and (b) one can't
be obtained from `CaughtException`. But with the accidental use of
`!=`, what it was actually doing was destroying the stack trace
supplied by the caller in the circumstance where `exception` was not a
`CaughtException`.

This should make it easier to debug some trybot failures that are
occurring in https://dart-review.googlesource.com/c/sdk/+/464245.

Change-Id: I6a6a6964072bf58db0bcddb22492b0a70203d43a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/464460
Auto-Submit: Paul Berry <paulberry@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

Author: stereotype441

pkg/analysis_server/lib/src/legacy_analysis_server.dart

@@ -829,7 +829,7 @@ class LegacyAnalysisServer extends AnalysisServer {
     bool fatal = false,
   }) {
     var msg = '$message: $exception';
-    if (stackTrace != null && exception is! CaughtException) {
+    if (stackTrace == null && exception is! CaughtException) {
       stackTrace = StackTrace.current;
     }