Add documentation on Snaps known errors.

snaps/how-to/communicate-errors.md

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+---
+description: Communicate errors from your Snap without crashing it
+sidebar_position: 9
+---
+
+# Communicate errors
+
+The Snaps SDK exposes a set of known errors that can be thrown from your Snap code without crashing the Snap.
+
+## Import and throw errors
+
+To throw these known errors, you have to import them from the `@metamask/snaps-sdk` package, then throw them where needed. Here's an example:
+
+```typescript
+import type { OnRpcRequestHandler } from '@metamask/snaps-sdk';
+import { MethodNotFoundError } from '@metamask/snaps-sdk';
+
+export const onRpcRequest: OnRpcRequestHandler = async ({ request }) => {
+  switch (request.method) {
+    case 'hello':
+      return 'Hello World!';
+    default:
+      // Throw a known error to avoid crashing the Snap
+      throw new MethodNotFoundError();
+  }
+};
+```
+
+### Pass data along with the error
+
+The error class constructors exported by `@metamask/snaps-sdk` have the following signature:
+
+```typescript
+class SnapJsonRpcError extends SnapError {
+  new (message?: string, data?: Record<string, Json>)
+}
+```
+
+Both parameters are optional. If you don't pass `message`, then a pre-determined message is used. If you don't pass `data`, then an empty object is passed.
+
+`data` can be any JSON-serializable object.
+
+## Detect known errors in Dapps
+
+Known errors are thrown back to the caller as JSON-RPC errors. They have a numeric `code`, a `message` string, and a `data` object.
+
+The [Snaps known errors reference](../reference/known-errors.md) lists all the known errors with their codes and intended usage.

snaps/reference/known-errors.md

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+---
+description: See the Snaps known errors reference
+sidebar_position: 7
+---
+
+# Snaps known errors
+
+## `InternalError`
+
+### Description
+This can be thrown by a Snap to indicate that an internal error occurred,
+without crashing the Snap.
+
+### Error code
+The code returned with this error is `-32603`.
+
+## `InvalidInputError`
+
+### Description
+This can be thrown by a Snap to indicate that the input to a method is
+invalid, without crashing the Snap.
+
+### Error code
+The code returned with this error is `-32000`.
+
+## `InvalidParamsError`
+
+### Description
+This can be thrown by a Snap to indicate that the parameters to a method are
+invalid, without crashing the Snap.
+
+### Error code
+The code returned with this error is `-32602`.
+
+## `InvalidRequestError`
+
+### Description
+This can be thrown by a Snap to indicate that the request is invalid, without
+crashing the Snap.
+
+### Error code
+The code returned with this error is `-32600`.
+
+## `LimitExceededError`
+
+### Description
+This can be thrown by a Snap to indicate that a limit has been exceeded,
+without crashing the Snap.
+
+### Error code
+The code returned with this error is `-32005`.
+
+## `MethodNotFoundError`
+
+### Description
+This can be thrown by a Snap to indicate that a method does not exist,
+without crashing the Snap.
+
+### Error code
+The code returned with this error is `-32601`.
+
+## `MethodNotSupportedError`
+
+### Description
+This can be thrown by a Snap to indicate that a method is not supported,
+without crashing the Snap.
+
+### Error code
+The code returned with this error is `-32004`.
+
+## `ParseError`
+
+### Description
+This can be thrown by a Snap to indicate that a request is not valid JSON,
+without crashing the Snap.
+
+### Error code
+The code returned with this error is `-32700`.
+
+## `ResourceNotFoundError`
+
+### Description
+This can be thrown by a Snap to indicate that a resource does not exist,
+without crashing the Snap.
+
+### Error code
+The code returned with this error is `-32001`.
+
+## `ResourceUnavailableError`
+
+### Description
+This can be thrown by a Snap to indicate that a resource is unavailable,
+without crashing the Snap.
+
+### Error code
+The code returned with this error is `-32002`.
+
+## `TransactionRejected`
+
+### Description
+This can be thrown by a Snap to indicate that a transaction was rejected,
+without crashing the Snap.
+
+### Error code
+The code returned with this error is `-32003`.
+
+## `ChainDisconnectedError`
+
+### Description
+This can be thrown by a Snap to indicate that the provider is disconnected
+from the requested chain, without crashing the Snap.
+
+### Error code
+The code returned with this error is `4901`.
+
+## `DisconnectedError`
+
+### Description
+This can be thrown by a Snap to indicate that the provider is disconnected,
+without crashing the Snap.
+
+### Error code
+The code returned with this error is `4900`.
+
+## `UnauthorizedError`
+
+### Description
+This can be thrown by a Snap to indicate that the requested method / account
+is not authorized by the user, without crashing the Snap.
+
+### Error code
+The code returned with this error is `4100`.
+
+## `UnsupportedMethodError`
+
+### Description
+This can be thrown by a Snap to indicate that the requested method is not
+supported by the provider, without crashing the Snap.
+
+### Error code
+The code returned with this error is `4200`.
+
+## `UserRejectedRequestError`
+
+### Description
+This can be thrown by a Snap to indicate that the user rejected the request,
+without crashing the Snap.
+
+### Error code
+The code returned with this error is `4001`.