Display warning when specified version was not found (#8155)

* Display warning message if specified package version isn't found
* made info a warning
* fixed version comparison and warning text that is displayed
* Addresses #4624

Author: rocklan

src/NuGetGallery.Services/PackageManagement/PackageFilter.cs

@@ -44,6 +44,8 @@ public Package GetFiltered(IReadOnlyCollection<Package> packages, PackageFilterC
                 result = result ?? _packageService.FilterLatestPackageBySuffix(packages, version, preRelease);
             }
 
+            // The package still wasn't found so let's just get the latest version. This applies when 
+            // no version is specified, or when the version asked for (in context.Version) wasn't found
             result =  result ?? _packageService.FilterLatestPackage(packages, SemVerLevelKey.SemVer2, allowPrerelease: true);
 
             return result;

src/NuGetGallery/Controllers/PackagesController.cs

@@ -869,6 +869,12 @@ public virtual async Task<ActionResult> DisplayPackage(string id, string version
                 model.GitHubDependenciesInformation = _contentObjectService.GitHubUsageConfiguration.GetPackageInformation(id);
             }
 
+            if (normalized != null && !string.Equals(package.NormalizedVersion, normalized, StringComparison.OrdinalIgnoreCase))
+            {
+                model.VersionRequested = normalized;
+                model.VersionRequestedWasNotFound = true;
+            }
+
             if (!string.IsNullOrWhiteSpace(package.LicenseExpression))
             {
                 try

src/NuGetGallery/ViewModels/PackageViewModel.cs

@@ -15,6 +15,8 @@ public class PackageViewModel : IPackageVersionModel
         public bool LatestStableVersionSemVer2 { get; set; }
         public bool DevelopmentDependency { get; set; }
         public bool Prerelease { get; set; }
+        public string VersionRequested { get; set; }
+        public bool VersionRequestedWasNotFound { get; set; }
         public int DownloadCount { get; set; }
         public bool Listed { get; set; }
         public bool FailedValidation { get; set; }

src/NuGetGallery/Views/Packages/DisplayPackage.cshtml

@@ -301,7 +301,14 @@
                     </text>
                 )
             }
-
+            @if (Model.VersionRequestedWasNotFound)
+            {
+                @ViewHelpers.AlertWarning(
+                    @<text>
+                        The specified version @Model.VersionRequested was not found. You have been taken to version @(Model.Version).
+                    </text>
+                )
+            }
             @if (Model.HasNewerRelease)
             {
                 @ViewHelpers.AlertInfo(