Improved team bref functions for error handling and robustness

[ajaviya]

The following PR aims to improve the robustness of team_pitching.py, team_fielding.py, and team_batting.py.

Currently, if a user inputs a valid three letter abbreviation for a team when calling team_[fielding, batting, or pitching]_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).

The following PR addresses both of these issues, raised in #462, by adding .upper() to the variable, team, and adding error handling for the season parameter range to increase the robustness of the respective function in each of the aforementioned files. It also improves the ValueError documentation for if start_season is None by including team as a parameter in the example calls of the function.