Improved documentation of exceptions in api.rst

[mhayworth]

Added use cases and inheritance information for the common types of exceptions that are listed in docs/api.rst. Documentation already exists for the main interface, request sessions, and lower-level classes on the website, so those may want to be moved into api.rst as well for consistent formatting. Alternatively, this information could also be chosen to move to the website. I am open to any changes we might want to make to improve this documentation and help users better understand the exceptions their code may be throwing.