Remove some NotImplementedExceptions/NotSupportedExceptions

ICSharpCode.Decompiler/CSharp/RequiredNamespaceCollector.cs

@@ -8,6 +8,7 @@ using System.Reflection.PortableExecutable;
 using System.Text;
 using System.Threading.Tasks;
 using ICSharpCode.Decompiler.Disassembler;
+using ICSharpCode.Decompiler.Metadata;
 using ICSharpCode.Decompiler.Semantics;
 using ICSharpCode.Decompiler.TypeSystem;
 using ICSharpCode.Decompiler.TypeSystem.Implementation;
@@ -102,8 +103,6 @@ namespace ICSharpCode.Decompiler.CSharp
 					CollectNamespaces(@event.AddAccessor, typeSystem, namespaces);
 					CollectNamespaces(@event.RemoveAccessor, typeSystem, namespaces);
 					break;
-				default:
-					throw new NotImplementedException();
 			}
 		}
 
@@ -195,7 +194,8 @@ namespace ICSharpCode.Decompiler.CSharp
 					case Metadata.OperandType.Sig:
 					case Metadata.OperandType.Tok:
 					case Metadata.OperandType.Type:
-						var handle = MetadataTokens.EntityHandle(instructions.ReadInt32());
+						var handle = MetadataTokenHelpers.EntityHandleOrNil(instructions.ReadInt32());
+						if (handle.IsNil) break;
 						switch (handle.Kind) {
 							case HandleKind.TypeDefinition:
 							case HandleKind.TypeReference:
@@ -218,8 +218,6 @@ namespace ICSharpCode.Decompiler.CSharp
 									}
 								}
 								break;
-							default:
-								throw new NotSupportedException();
 						}
 						break;
 					default:
@@ -250,8 +248,6 @@ namespace ICSharpCode.Decompiler.CSharp
 					foreach (var arg in method.TypeArguments)
 						CollectNamespacesForTypeReference(arg, namespaces);
 					break;
-				default:
-					throw new NotImplementedException();
 			}
 		}
 	}

ICSharpCode.Decompiler/IL/Transforms/DynamicCallSiteTransform.cs

@@ -237,9 +237,8 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 						operand: targetInvokeCall.Arguments[2]
 					);
 				default:
-					throw new NotSupportedException();
+					throw new ArgumentOutOfRangeException($"Value {callsite.Kind} is not supported!");
 			}
-			throw new NotImplementedException();
 		}
 
 		bool ScanCallSiteInitBlock(Block callSiteInitBlock, IField callSiteCacheField, IType callSiteDelegateType, out CallSiteInfo callSiteInfo, out Block blockAfterInit)

ICSharpCode.Decompiler/TypeSystem/MetadataAssembly.cs

@@ -295,7 +295,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		{
 			var method = GetDefinition(methodDefHandle);
 			if (method == null) {
-				throw new NotImplementedException();
+				throw new BadImageFormatException("MethodDef not found in current assembly.");
 			}
 			if (expandVarArgs && method.Parameters.LastOrDefault()?.Type.Kind == TypeKind.ArgList) {
 				method = new VarArgInstanceMethod(method, EmptyList<IType>.Instance);