Improve documentation, add questions section

Signed-off-by: Eli Skeggs <skeggse@gmail.com>
Error callbacks now include context

Signed-off-by: Eli Skeggs <skeggse@gmail.com>
Add param if specified on error

Signed-off-by: Eli Skeggs <skeggse@gmail.com>
Move common checks to baseCheck

Signed-off-by: Eli Skeggs <skeggse@gmail.com>
Update tests for new common secondary error parameter

Signed-off-by: Eli Skeggs <skeggse@gmail.com>