Added test coverage on exceptions. (#18)

Signed-off-by: AraHaan <seandhunt_7@yahoo.com>

Author: AraHaan

src/GenerationFailedException.cs

@@ -78,7 +78,7 @@ private static Generator Create(string optionsText, string gitInfoText)
             };
             if (string.IsNullOrEmpty(generator.options!.AssemblyType))
             {
-                throw new GenerationFailedException("AssemblyType should not be null or an empty string.");
+                throw new InvalidOperationException("AssemblyType should not be null or an empty string.");
             }
 
             return generator;
@@ -150,4 +150,4 @@ private SyntaxNodeOrToken[] MakeAttributeArgumentList(string[] args, string type
             return lst;
         }
     }
-}
+}

src/Generator.cs

@@ -1,5 +1,6 @@
 namespace GitBuildInfo.SourceGenerator.Tests
 {
+    using System;
     using GitBuildInfo.SourceGenerator;
     using Xunit;
 
@@ -8,7 +9,7 @@ public class GeneratorTests
         [Fact]
         public void TestGeneratingNonGeneric()
         {
-            var result = this.DoTest("TestNamespace.Test", false);
+            var result = DoTest("TestNamespace.Test", false);
             Assert.Equal($@"// <autogenerated/>
 using Elskom.Generic.Libs;
 using TestNamespace;
@@ -20,7 +21,7 @@ public void TestGeneratingNonGeneric()
         [Fact]
         public void TestGeneratingGeneric()
         {
-            var result = this.DoTest("TestNamespace.Test", true);
+            var result = DoTest("TestNamespace.Test", true);
             Assert.Equal($@"// <autogenerated/>
 using Elskom.Generic.Libs;
 using TestNamespace;
@@ -31,13 +32,11 @@ public void TestGeneratingGeneric()
 
         [Fact]
         public void TestGeneratingFailure()
-        {
-            Assert.Throws<GenerationFailedException>(() => this.DoTest(string.Empty, false));
-        }
+            => Assert.Throws<InvalidOperationException>(() => DoTest(string.Empty, false));
 
         // the values on GitInfo are not valid in terms of results
         // that would normally be from git.
-        private string DoTest(string assemblyType, bool generic)
+        private static string DoTest(string assemblyType, bool generic)
             => Generator.CreateAndGenerateCode(
                 $@"{{
   ""$schema"": ""https://raw.githubusercontent.com/Elskom/GitBuildInfo.SourceGenerator/main/settings.schema.json"",