Add Errors page to dart (#236)

Add `Errors` page to dart

---------

Signed-off-by: Sri Krishna <[email protected]>

docs/dart/errors.md

@@ -0,0 +1,119 @@
+---
+title: Errors
+sidebar_position: 4
+---
+
+Similar to the familiar "404 Not Found" and "500 Internal Server Error" status
+codes you may have seen in HTTP, Connect uses a set of [16 error codes](/docs/protocol#error-codes).
+In the Connect protocol, an error is always represented as JSON, and is easily
+readable in the developer tools of your browser. For example:
+
+```
+HTTP/1.1 400 Bad Request
+Content-Type: application/json
+
+{
+  "code": "invalid_argument",
+  "message": "sentence cannot be empty"
+}
+```
+
+With the gRPC/gRPC-Web protocols, errors are usually not human-readable, but
+Connect provides a common type that represents errors consistently across
+all supported protocols.
+
+## Working with errors
+
+All errors are represented by [`ConnectException`][exception], a subtype of the built-in `Exception` class. Using a try-catch block, we can catch any error that occurred during a call:
+
+```dart
+import 'package:connectrpc/connect.dart';
+
+try {
+  await client.say(SayRequest());
+} catch (ex) {
+  // We have to verify ex is a ConnectException
+  // before using it as one.
+  if (ex is ConnectException) {
+    ex.code;  // Code.invalidArgument
+    ex.message; // "sentence cannot be empty"
+  }
+  // Alternatively, we can use ConnectException.from()
+  // It returns a ConnectException as is, and converts any
+  // other error to a ConnectException.
+  final connectEx = ConnectException.from(ex);
+  connectEx.code; // Code.invalidArgument
+  connectEx.message; // "sentence cannot be empty"
+}
+```
+
+## Error codes
+
+The `code` property holds one of Connect's [error codes](/docs/protocol#error-codes).
+All error codes are available through the enumeration [`Code`][code].
+
+Note that a `code` has both the integer value and string representation.
+
+```dart
+import 'package:connectrpc/connect.dart';
+
+final code = Code.invalidArgument;
+code.value; // 3
+code.name; // "invalid_argument"
+```
+
+## Error messages
+
+The `message` property contains a descriptive error message. In most cases,
+the message is provided by the backend implementing the service.
+
+## Metadata
+
+If you catch an error, your program takes an exception from the regular code
+path, but you might still want to access a header or trailer value. Connect
+provides a union of header and trailer values in the `metadata` property as a
+simple [`Headers`][headers] object:
+
+```dart
+ex.metadata["custom-header-value"];
+ex.metadata["custom-trailer-value"];
+```
+
+## Error details
+
+On the wire, error details are wrapped with `google.protobuf.Any`, so that a
+server or middleware can attach arbitrary data to an error.
+
+This example looks up a localized error message in the users preferred
+language:
+
+```dart
+import 'package:connectrpc/connect.dart';
+import "./gen/google/rpc/error_details_pb";
+
+String getMessageForLocale(ConnectException ex, String locale) {
+  final localized = LocalizedMessage();
+  for (final detail in ex.details) {
+    if (detail.type != localized.info_.qualifiedMessageName) {
+      continue;
+    }
+    localized.clear();
+    localized.mergeFromBuffer(detail.value);
+    if (localized.locale != locale) {
+      continue;
+    }
+    return localized.message;
+  }
+  return ex.message;
+}
+```
+
+We are using the protobuf message [`google.rpc.LocalizedMessage`][localized-message]
+in this example - run `buf generate buf.build/googleapis/googleapis` to
+generate this message - but any Protobuf message can be transmitted as error
+details.
+
+[code]: https://github.com/connectrpc/connect-dart/blob/main/packages/connect/lib/src/code.dart#L22
+[exception]: https://github.com/connectrpc/connect-dart/blob/main/packages/connect/lib/src/exception.dart#L30
+[headers]: https://github.com/connectrpc/connect-dart/blob/main/packages/connect/lib/src/headers.dart#L18
+[localizaed-message]: https://buf.build/googleapis/googleapis/file/main:google/rpc/error_details.proto#L241