Implemented extern functions

src/errors/Common.go

@@ -9,6 +9,7 @@ var (
 	ExpectedIfBeforeElse       = &Base{"Expected an 'if' block before 'else'"}
 	ExpectedPackageName        = &Base{"Expected package name"}
 	ExpectedStructName         = &Base{"Expected struct name"}
+	ExpectedDLLName            = &Base{"Expected DLL name"}
 	InvalidNumber              = &Base{"Invalid number"}
 	InvalidExpression          = &Base{"Invalid expression"}
 	InvalidRune                = &Base{"Invalid rune"}