Change Error to Exception

[werainkhatri]

Describe the bug
Custom Errors classes have been created and caught where Exceptions are to be created and caught.

To Reproduce
Its a semantic bug, cannot be produced.

Expected behavior
An Exception in Dart should be thrown for regular, expected program flow and is intended to be caught, whereas An Error in Dart should be thrown for unexpected program flow and should not be caught but addressed by the programmer. So, Instead of extending Error, we should implement Exception.