Emit exit code 69 for TlsExceptions (#1730)

Closes #1729

Author: nex3

lib/src/command_runner.dart

@@ -235,6 +235,7 @@ and include the logs in an issue on https://github.com/dart-lang/pub/issues/new
     if (exception is HttpException ||
         exception is http.ClientException ||
         exception is SocketException ||
+        exception is TlsException ||
         exception is PubHttpException ||
         exception is DependencyNotFoundException) {
       return exit_codes.UNAVAILABLE;

lib/src/source/hosted.dart

@@ -327,15 +327,16 @@ class BoundHostedSource extends CachedSource {
           "to find package $package at $url.",
           error,
           stackTrace);
-    }
-
-    if (error is io.SocketException) {
+    } else if (error is io.SocketException) {
       fail("Got socket error trying to find package $package at $url.", error,
           stackTrace);
+    } else if (error is io.TlsException) {
+      fail("Got TLS error trying to find package $package at $url.", error,
+          stackTrace);
+    } else {
+      // Otherwise re-throw the original exception.
+      throw error;
     }
-
-    // Otherwise re-throw the original exception.
-    throw error;
   }
 
   /// Given a URL, returns a "normalized" string to be used as a directory name