[DOC] add dedicated page on error handling

closes #632

doc/reference/error_handling.md

@@ -0,0 +1,81 @@
+# 🦉 Error Handling 🦉
+
+## Content
+
+- [Overview](#overview)
+- [Example](#example)
+- [Reference](#reference)
+
+## Overview
+
+By default, whenever an error occurs in the rendering of an Owl application, we
+destroy the whole application. Otherwise, we cannot offer any guarantee on the
+state of the resulting component tree. It might be hopelessly corrupted, but
+without any user-visible state.
+
+Clearly, it sometimes is a little bit extreme to destroy the application. This
+is why we have a builtin mechanism to handle rendering errors (and errors coming
+from lifecycle hooks): the `catchError` hook.
+
+## Example
+
+For example, here is how we could implement an `ErrorBoundary` component:
+
+```xml
+<div t-name="ErrorBoundary">
+    <t t-if="state.error">
+        Error handled
+    </t>
+    <t t-else="">
+        <t t-slot="default" />
+    </t>
+</div>
+```
+
+```js
+class ErrorBoundary extends Component {
+  state = useState({ error: false });
+
+  catchError() {
+    this.state.error = true;
+  }
+}
+```
+
+Using the `ErrorBoundary` is then extremely simple:
+
+```xml
+<ErrorBoundary><SomeOtherComponent/></ErrorBoundary>
+```
+
+Note that we need to be careful here: the fallback UI should not throw any
+error, otherwise we risk going into an infinite loop (also, see the page on
+[slots](slots.md) for more information on the `t-slot` directive).
+
+## Reference
+
+Whenever the `catchError` lifecycle hook is implemented, all errors coming from
+sub components rendering and/or lifecycle method calls will be caught and given
+to the `catchError` method. This allows us to properly handle the error, and to
+not break the application.
+
+There are important things to know:
+
+- If an error that occured in the internal rendering cycle is not caught, then
+  Owl will destroy the full application. This is done on purpose, because Owl
+  cannot guarantee that the state is not corrupted from this point on.
+
+- errors coming from event handlers are NOT managed by `catchError` or any other
+  owl mechanism. This is up to the application developer to properly recover
+  from an error
+
+Also, it may be useful to know that whenever an error is caught, it is then
+broadcasted to the application by an event on the `qweb` instance. It may be
+useful, for example, to log the error somewhere.
+
+```js
+env.qweb.on("error", null, function(error) {
+  // do something
+  // react to the error
+});
+```