Improve error messages to include action name

[cmbrandenburg]

Currently (as of v0.2.0), when an action fails, the resulting error message does not include the action name. Consequently, if the application runs many actions and one fails, then it's not immediately clear which action failed.

[cmbrandenburg]

As part of resolving this issue, Chill's Error enum should become an opaque type and applications should instead use a new ErrorKind enum for distinguishing between different CouchDB errors such as, e.g., “not found” vs “document conflict.” This change would free the Error type to become a struct that would contain common fields for all error kinds, including something like an Option<&'static str> for describing the action that failed.