[rest api]: improve exception handling for `/connect` and add `content-type` header for `/download`

This PR achieves the following:
1. Improves the exception handling when dialling a peer fails or an unknown error occurs.
2. Add a `Content-Type` header to the `/download` endpoint of `application/octet-stream`, which is [defined by MDN](https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types#types) as meant to be used for "generic binary data (or binary data whose true type is unknown)".

Co-authored-by: Michael Bradley <michaelsbradleyjr@gmail.com>

dagger/rest/api.nim

@@ -86,9 +86,14 @@ proc initRestApi*(node: DaggerNodeRef): RestRouter =
             peerRecord.get().addresses.mapIt(
               it.address
             )
+      try:
+        await node.connect(peerId.get(), addresses)
+        return RestApiResponse.response("Successfully connected to peer")
+      except DialFailedError as e:
+        return RestApiResponse.error(Http400, "Unable to dial peer")
+      except CatchableError as e:
+        return RestApiResponse.error(Http400, "Unknown error dialling peer")
 
-      await node.connect(peerId.get(), addresses)
-      return RestApiResponse.response("")
 
   router.api(
     MethodGet,
@@ -113,6 +118,7 @@ proc initRestApi*(node: DaggerNodeRef): RestRouter =
             retr.isErr):
             return RestApiResponse.error(Http404, retr.error.msg)
 
+        resp.addHeader("Content-Type", "application/octet-stream")
         await resp.prepareChunked()
         while not stream.atEof:
           var