Implemented assert keyword

src/build/errors/CompileErrors.go

@@ -5,6 +5,7 @@ var (
 	InvalidExpression   = &Base{"Invalid expression"}
 	InvalidRune         = &Base{"Invalid rune"}
 	InvalidStatement    = &Base{"Invalid statement"}
+	MissingExpression   = &Base{"Missing expression"}
 	MissingMainFunction = &Base{"Missing main function"}
 	MissingOperand      = &Base{"Missing operand"}
 	NotImplemented      = &Base{"Not implemented"}