Small friends fixes (#4270)

* Ensure that fetch errors are properly propagated

* Handle user not found errors better in add_friend

* Cargo fmt

* Introduce new LabrinthError returnable by fetch_advanced

* Allow enter key to send a friend request

apps/app-frontend/src/components/ui/friends/FriendsList.vue

@@ -250,7 +250,13 @@ onUnmounted(() => {
 		<div class="mb-4">
 			<h2 class="m-0 text-lg font-extrabold text-contrast">Username</h2>
 			<p class="m-0 mt-1 leading-tight">You can add friends with their Modrinth username.</p>
-			<input v-model="username" class="mt-2 w-full" type="text" placeholder="Enter username..." />
+			<input
+				v-model="username"
+				class="mt-2 w-full"
+				type="text"
+				placeholder="Enter username..."
+				@keyup.enter="addFriendFromModal"
+			/>
 		</div>
 		<ButtonStyled color="brand">
 			<button :disabled="username.length === 0" @click="addFriendFromModal">