fix: add missing toString() overrides to Exceptions

lib/src/core/consumed_thing.dart

@@ -29,6 +29,11 @@ class UnexpectedReponseException implements Exception {
 
   /// Creates a new [UnexpectedReponseException] from an error [message].
   UnexpectedReponseException(this.message);
+
+  @override
+  String toString() {
+    return message;
+  }
 }
 
 /// Implementation of the [scripting_api.ConsumedThing] interface.

lib/src/core/content.dart

@@ -18,6 +18,11 @@ class ContentDeserializationException implements Exception {
 
   /// Creates a new [ContentDeserializationException] with an error [message].
   ContentDeserializationException(this.message, this.error);
+
+  @override
+  String toString() {
+    return "$runtimeType: $message";
+  }
 }
 
 void _onError(Object error) {

lib/src/core/content_serdes.dart

@@ -37,6 +37,11 @@ class ContentSerdesException implements Exception {
 
   /// Constructor.
   ContentSerdesException(this.message);
+
+  @override
+  String toString() {
+    return "$runtimeType: $message";
+  }
 }
 
 /// Class providing serializing and deserializing capabilities.

lib/src/core/wot.dart

@@ -14,14 +14,17 @@ import 'thing_discovery.dart' show ThingDiscovery;
 /// This [Exception] is thrown if an error during the consumption of a
 /// [ThingDescription] occurs.
 class ThingConsumptionException implements Exception {
-  /// The actual error message.
-  String message;
-
   /// The identifier of the [ThingDescription] that triggered this [Exception].
-  String identifier;
+  final String identifier;
 
   /// Constructor
-  ThingConsumptionException(this.message, this.identifier);
+  ThingConsumptionException(this.identifier);
+
+  @override
+  String toString() {
+    return "$runtimeType: A ConsumedThing with identifier $identifier already "
+        "exists.";
+  }
 }
 
 /// Implementation of the [scripting_api.WoT] runtime interface.
@@ -45,8 +48,7 @@ class WoT implements scripting_api.WoT {
       return newThing;
     } else {
       final id = thingDescription.identifier;
-      throw ThingConsumptionException(
-          "A ConsumedThing with identifier $id already exists", id);
+      throw ThingConsumptionException(id);
     }
   }
 

lib/src/definitions/form.dart

@@ -414,4 +414,9 @@ class UriVariableException implements Exception {
 
   /// Constructor.
   UriVariableException(this.message);
+
+  @override
+  String toString() {
+    return "$runtimeType: $message";
+  }
 }