Update async/await guideline to not break other guideline (#6559)

Resolves #3132

Author: parlough

examples/misc/lib/effective_dart/usage_bad.dart

@@ -315,7 +315,7 @@ class BadTeam extends Team {
             return players.where((player) => player.isActive).length;
           });
         })
-        .catchError((e) {
+        .onError<DownloadException>((e, _) {
           log.error(e);
           return 0;
         });

examples/misc/lib/effective_dart/usage_good.dart

@@ -319,6 +319,8 @@ class Player {
   bool get isActive => false;
 }
 
+class DownloadException implements Exception {}
+
 class Team {
   Future<List<Player>> get roster => Future.value([]);
   Future<Team?> downloadTeam(String name) => Future.value(Team());
@@ -332,7 +334,7 @@ class Team {
 
       var players = await team.roster;
       return players.where((player) => player.isActive).length;
-    } catch (e) {
+    } on DownloadException catch (e, _) {
       log.error(e);
       return 0;
     }

src/content/effective-dart/usage.md

@@ -1564,7 +1564,7 @@ Future<int> countActivePlayers(String teamName) [!async!] {
 
     var players = [!await!] team.roster;
     return players.where((player) => player.isActive).length;
-  } catch (e) {
+  } on DownloadException catch (e, _) {
     log.error(e);
     return 0;
   }
@@ -1582,7 +1582,7 @@ Future<int> countActivePlayers(String teamName) {
           return players.where((player) => player.isActive).length;
         });
       })
-      .catchError((e) {
+      .onError<DownloadException>((e, _) {
         log.error(e);
         return 0;
       });