Write tests for erroronous code.

[srijan-paul]

Currently, the unit tests only verify if a certain piece of code executes successfully and has the expected behavior.
However, often times the "expected behavior" is an error. e.g: The expected behavior for1 + "2" to report a bad binary operator error and stop execution.

We should have tests that assert that the compiler/runtime is able to report correct and well formed errors and doesn't crash/segfault.

[srijan-paul]

This is being worked on as of #23