team_fielding_bref() can be improved for better parameter formatting and error handling

If a user inputs a valid three letter abbreviation for a team when calling team_fielding_bref(), but without all letters capitalized (ex. "oak" or "OaK" instead of "OAK"), it raises the error: `AttributeError: 'NoneType' object has no attribute 'find'`

Additionally, there is error handling if a user does not input a start_season, but no error handling if a user inputs an invalid range (ex. end season before start season).

I would like to propose adding .upper() to the variable, team, and adding error handling for the season parameter range to increase the robustness of this function.