Note in UPGRADE.md about new base class of GraphQL\Error\UserError

Author: vladar

UPGRADE.md

@@ -1,7 +1,7 @@
 # Upgrade
 
 ## Upgrade v0.9.x > v0.10.x
-#### Breaking: default error formatting
+### Breaking: default error formatting
 By default exceptions thrown in resolvers will be reported with generic message `"Internal server error"`.
 Only exceptions implementing interface `GraphQL\Error\ClientAware` and claiming themselves as `safe` will 
 be reported with full error message.
@@ -26,18 +26,30 @@ GraphQL\Error\FormattedError::setInternalErrorMessage("Unexpected error");
 **This change only affects default error reporting mechanism. If you set your own error formatter using 
 `ExecutionResult::setErrorFormatter()` you won't be affected by this change.**
 
-If you are not happy with this change just set [your own error 
+If default formatting doesn't work for you - just set [your own error 
 formatter](http://webonyx.github.io/graphql-php/error-handling/#custom-error-formatting).
 
-#### Deprecated: GraphQL\Utils moved to GraphQL\Utils\Utils
+### Breaking: `GraphQL\Error\UserError` base class
+`GraphQL\Error\UserError` now extends `\RuntimeException` (previously it was extending 
+`GraphQL\Error\InvariantViolation`).
+
+**This may affect authors of derived libraries and those using custom error formatting.** 
+If you were catching `InvariantViolation` anywhere in your code, you should also catch `UserError` now.
+
+Same applies to `instanceof` checks.
+
+`UserError` is thrown when library detects invalid input from client.
+
+
+### Deprecated: `GraphQL\Utils` moved to `GraphQL\Utils\Utils`
 Old class still exists, but triggers deprecation warning when referenced.
 
 ## Upgrade v0.7.x > v0.8.x
 All of those changes apply to those who extends various parts of this library.
 If you only use the library and don't try to extend it - everything should work without breaks.
 
 
-#### Breaking: Custom directives handling
+### Breaking: Custom directives handling
 When passing custom directives to schema, default directives (like `@skip` and `@include`) 
 are not added to schema automatically anymore. If you need them - add them explicitly with 
 your other directives.
@@ -58,15 +70,15 @@ $schema = new Schema([
 ]);
 ```
 
-#### Breaking: Schema protected property and methods visibility 
+### Breaking: Schema protected property and methods visibility 
 Most of the `protected` properties and methods of `GraphQL\Schema` were changed to `private`.
 Please use public interface instead.
 
-#### Breaking: Node kind constants
+### Breaking: Node kind constants
 Node kind constants were extracted from `GraphQL\Language\AST\Node` to 
 separate class `GraphQL\Language\AST\NodeKind`
 
-#### Non-breaking: AST node classes renamed
+### Non-breaking: AST node classes renamed
 AST node classes were renamed to disambiguate with types. e.g.:
 
 ```
@@ -78,7 +90,7 @@ etc.
 Old names are still available via `class_alias` defined in `src/deprecated.php`. 
 This file is included automatically when using composer autoloading.
 
-#### Deprecations
+### Deprecations
 There are several deprecations which still work, but trigger `E_USER_DEPRECATED` when used.
 
 For example `GraphQL\Executor\Executor::setDefaultResolveFn()` is renamed to `setDefaultResolver()`
@@ -89,7 +101,7 @@ but still works with old name.
 There are a few new breaking changes in v0.7.0 that were added to the graphql-js reference implementation 
 with the spec of April2016
 
-#### 1. Context for resolver
+### 1. Context for resolver
 
 You can now pass a custom context to the `GraphQL::execute` function that is available in all resolvers as 3rd argument. 
 This can for example be used to pass the current user etc.
@@ -147,7 +159,7 @@ function resolveMyField($object, array $args, $context, ResolveInfo $info){
 }
 ```
 
-#### 2. Schema constructor signature
+### 2. Schema constructor signature
 
 The signature of the Schema constructor now accepts an associative config array instead of positional arguments:
 
@@ -165,7 +177,7 @@ $schema = new Schema([
 ]);
 ```
 
-#### 3. Types can be directly passed to schema
+### 3. Types can be directly passed to schema
 
 There are edge cases when GraphQL cannot infer some types from your schema. 
 One example is when you define a field of interface type and object types implementing