Throw if we can't add/remove PackageReferences from the project

src/VisualStudio/IntegrationTest/TestUtilities/InProcess/SolutionExplorer_InProc.cs

@@ -229,6 +229,10 @@ public void AddPackageReference(string projectName, string packageName, string v
                     return await packageService.AddAsync(packageName, version);
                 });
             }
+            else
+            {
+                throw new InvalidOperationException($"'{nameof(AddPackageReference)}' is not supported in project '{projectName}'.");
+            }
         }
 
         public void RemovePackageReference(string projectName, string packageName)
@@ -245,6 +249,10 @@ public void RemovePackageReference(string projectName, string packageName)
                     return await packageService.RemoveAsync(packageName);
                 });
             }
+            else
+            {
+                throw new InvalidOperationException($"'{nameof(RemovePackageReference)}' is not supported in project '{projectName}'.");
+            }
         }
 
         public void RemoveProjectReference(string projectName, string projectReferenceName)