Adding exceptions

[Antika7]

Issue Resolved : Link

Description -

Replace generic exceptions with typed custom exceptions

Summary

Replaces all 9 bare raise Exception(...) calls in init.py with typed exceptions from rbiparser.exceptions, making errors easier to catch and handle programmatically.

Changes

exceptions.py

created class to include exceptions

init.py

Function	Exception raised
get_sheet_urls – bad HTTP status	DownloadError
get_sheet_urls – no .xlsx links found	ParseError
convert_xlsx_to_csv – workbook can't be opened	ParseError
convert_xlsx_to_csv – header column mismatch	ParseError
convert_xlsx_to_csv – sheet conversion fails	ParseError
save_etags – file write failure	StorageError
get_url_headers – network error	DownloadError
download – fetch failure	DownloadError
download – disk write failure	StorageError

Bug fixes (incidental)

Four of the original raises used the multi-argument form Exception("msg", var, ...), which stores a tuple in args[0] rather than a human-readable string. All are now f-strings.

Why

Callers can now distinguish between a network failure (DownloadError), a malformed file (ParseError), and an I/O problem (StorageError) without parsing exception message strings. All three are subclasses of RBIParserError, so existing broad except RBIParserError handlers are unaffected.

[Antika7]

@knadh @vividvilla Please review this fix for issue 15

Kindly review and merge accordingly