Make ErrorList introspectable

[lavalamp]

From discussion in #1593:

• Delete the ErrorList.ToError() function
• Remove errorListInternal, and move its Error() function to be on ErrorList
• Make a func ToError(errs []error) error function that returns nil if errs is nil, otherwise an ErrorList

I don't feel bad about making folks pass around []error until they want to collapse into a single error, because ErrorList is not really a helpful type at the moment-- it's actually more typing than []error and doesn't offer any features besides ToError.