Separate program and user errors

[komninoschatzipapas]

There are two kinds of errors:

1. User errors: Thrown when a user(programmer) provides invalid code. For example, forgetting a semi colon between statements.
2. Program errors: Thrown when something in the actual interpreter program goes wrong. For example reaching a clause that shouldn't be reached.

In the current implementation there is no separation between these two kinds of errors. For user errors, a PSIError is always thrown but for program errors, sometimes a PSIError and sometimes an Error is thrown.

Modify the error module and related modules so a PSIError can always be thrown and add separation.

Also modify the psi module so correct error messages are displayed.