improve mod update-check validation & errors (#336)

src/StardewModdingAPI.Web/Controllers/ModsController.cs

@@ -106,14 +106,14 @@ namespace StardewModdingAPI.Web.Controllers
                 // parse mod key
                 if (!this.TryParseModKey(modKey, out string vendorKey, out string modID))
                 {
-                    result[modKey] = new ModInfoModel("The mod key isn't in a valid format. It should contain the mod repository key and mod ID like 'Nexus:541'.");
+                    result[modKey] = new ModInfoModel("The mod key isn't in a valid format. It should contain the site key and mod ID like 'Nexus:541'.");
                     continue;
                 }
 
                 // get matching repository
                 if (!this.Repositories.TryGetValue(vendorKey, out IModRepository repository))
                 {
-                    result[modKey] = new ModInfoModel("There's no mod repository matching this namespaced mod ID.");
+                    result[modKey] = new ModInfoModel($"There's no mod site with key '{vendorKey}'. Expected one of [{string.Join(", ", this.Repositories.Keys)}].");
                     continue;
                 }
 

src/StardewModdingAPI.Web/Framework/ModRepositories/ChucklefishRepository.cs

@@ -43,6 +43,11 @@ namespace StardewModdingAPI.Web.Framework.ModRepositories
         /// <param name="id">The mod ID in this repository.</param>
         public override async Task<ModInfoModel> GetModInfoAsync(string id)
         {
+            // validate ID format
+            if (!uint.TryParse(id, out uint _))
+                return new ModInfoModel($"The value '{id}' isn't a valid Chucklefish mod ID, must be an integer ID.");
+
+            // fetch info
             try
             {
                 // fetch HTML

src/StardewModdingAPI.Web/Framework/ModRepositories/GitHubRepository.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Net;
 using System.Threading.Tasks;
 using Newtonsoft.Json;
 using Pathoschild.Http.Client;
@@ -46,6 +47,11 @@ namespace StardewModdingAPI.Web.Framework.ModRepositories
         /// <param name="id">The mod ID in this repository.</param>
         public override async Task<ModInfoModel> GetModInfoAsync(string id)
         {
+            // validate ID format
+            if (!id.Contains("/") || id.IndexOf("/", StringComparison.InvariantCultureIgnoreCase) != id.LastIndexOf("/", StringComparison.InvariantCultureIgnoreCase))
+                return new ModInfoModel($"The value '{id}' isn't a valid GitHub mod ID, must be a username and project name like 'Pathoschild/LookupAnything'.");
+
+            // fetch info
             try
             {
                 GitRelease release = await this.Client
@@ -53,6 +59,10 @@ namespace StardewModdingAPI.Web.Framework.ModRepositories
                     .As<GitRelease>();
                 return new ModInfoModel(id, this.NormaliseVersion(release.Tag), $"https://github.com/{id}/releases");
             }
+            catch (ApiException ex) when (ex.Status == HttpStatusCode.NotFound)
+            {
+                return new ModInfoModel("Found no mod with this ID.");
+            }
             catch (Exception ex)
             {
                 return new ModInfoModel(ex.ToString());

src/StardewModdingAPI.Web/Framework/ModRepositories/NexusRepository.cs

@@ -38,6 +38,11 @@ namespace StardewModdingAPI.Web.Framework.ModRepositories
         /// <param name="id">The mod ID in this repository.</param>
         public override async Task<ModInfoModel> GetModInfoAsync(string id)
         {
+            // validate ID format
+            if (!uint.TryParse(id, out uint _))
+                return new ModInfoModel($"The value '{id}' isn't a valid Nexus mod ID, must be an integer ID.");
+
+            // fetch info
             try
             {
                 NexusResponseModel response = await this.Client