Pass correct type to error message for unsupported field types.

Author: Kristoffer Lerbæk Pedersen

src/Protobuf.System.Text.Json/FieldTypeResolver.cs

@@ -58,7 +58,7 @@ public static Type ResolverFieldType(FieldDescriptor fieldDescriptor, Dictionary
                 return propertyTypeLookup[fieldDescriptor.PropertyName];
             default:
                 throw new ArgumentOutOfRangeException(nameof(fieldDescriptor),
-                    $"FieldType: '{fieldDescriptor}' is not supported.");
+                    $"FieldType: '{fieldDescriptor.FieldType}' is not supported.");
         }
     }
 }