feat(core): improve error handling for not found plugin assemblies

src/Core/Exceptions/PluginNotFoundException.cs

@@ -0,0 +1,19 @@
+using System;
+
+namespace CPlugin.Net;
+
+/// <summary>
+/// The exception that is thrown when a plugin assembly cannot be found.
+/// </summary>
+public class PluginNotFoundException : Exception
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="PluginNotFoundException"/> class.
+    /// </summary>
+    /// <param name="message">
+    /// The message that describes the error.
+    /// </param>
+    public PluginNotFoundException(string message) : base(message)
+    {
+    }
+}

src/Core/PluginLoadContext.cs

@@ -10,6 +10,20 @@ internal class PluginLoadContext : AssemblyLoadContext
 
     public PluginLoadContext(string pluginPath)
     {
+        if (!File.Exists(pluginPath))
+        {
+            var message =
+            $"""
+             The plugin '{Path.GetFileName(pluginPath)}' was not found at:
+
+             {pluginPath}
+
+             Ensure the plugin project has been built and the assembly exists in the plugins directory.
+             """;
+
+            throw new PluginNotFoundException(message);
+        }
+
         _resolver = new AssemblyDependencyResolver(pluginPath);
     }
 

src/Core/PluginLoader.cs

@@ -33,6 +33,9 @@ public static class PluginLoader
     /// <exception cref="PluginDependencyException">
     /// A plugin declares a dependency that cannot be resolved.
     /// </exception>
+    /// <exception cref="PluginNotFoundException">
+    /// A configured plugin assembly could not be found.
+    /// </exception>
     public static void Load(CPluginConfigurationBase configuration)
     {
         ArgumentNullException.ThrowIfNull(configuration);

tests/CPlugin.Net/Core/PluginLoaderTests.cs

@@ -129,4 +129,21 @@ public void Load_WhenPluginDependencyIsResolved_ShouldLoadPluginsSuccessfully()
         string weapons = gameModes[0].ExecuteAction();
         weapons.Should().Be("Pistol, AK-47");
     }
+
+    [Test]
+    public void Load_WhenPluginIsNotFound_ShouldThrowPluginNotFoundException()
+    {
+        // Arrange
+        var value = "Example.EconomyPlugin.dll";
+        Environment.SetEnvironmentVariable("PLUGINS", value);
+        var configuration = new CPluginEnvConfiguration();
+
+        // Act
+        Action act = () => PluginLoader.Load(configuration);
+
+        // Assert
+        act.Should()
+           .Throw<PluginNotFoundException>()
+           .WithMessage("*Example.EconomyPlugin.dll*");
+    }
 }