InspectorProxy: Don't pass an Error object to socket.close()

Summary:
Currently our error handling for websocket connections in `InspectorProxy` passes whatever is thrown as an "error explanation" to `socket.close()`.

This argument expects `string | undefined` (NB `null` is also invalid)

In cases where we're catching and passing an `Error`, this causes the socket instance to emit its own `'error'`, which we don't have any listener for, causing a crash.

This diff uses `e?.toString() ?? 'Unknown error'` to:
 - Preserve the current behaviour for raw strings
 - Show `Error: ${e.message}` when `e instanceof Error`
 - Show `Unknown error` when `e == null`.

Changelog:
**Fix:** Prevent `InspectorProxy` connection errors crashing the process.

Reviewed By: motiz88

Differential Revision: D36479972

fbshipit-source-id: 622be8c5f6bc516dd3bf9115445c7fca9c841409

packages/metro-inspector-proxy/src/InspectorProxy.js

@@ -177,7 +177,7 @@ class InspectorProxy {
         });
       } catch (e) {
         console.error('error', e);
-        socket.close(INTERNAL_ERROR_CODE, e);
+        socket.close(INTERNAL_ERROR_CODE, e?.toString() ?? 'Unknown error');
       }
     });
     return wss;
@@ -212,7 +212,7 @@ class InspectorProxy {
         device.handleDebuggerConnection(socket, pageId);
       } catch (e) {
         console.error(e);
-        socket.close(INTERNAL_ERROR_CODE, e);
+        socket.close(INTERNAL_ERROR_CODE, e?.toString() ?? 'Unknown error');
       }
     });
     return wss;