Make constants of constant error messages

AFAIK this seem to be best practice Golang-wise.
Document potential errors
fix(typo): ErrNotEventsFound => ErrNoEventsFound

Discussed this with @maxpersson in #8.
fix: rename NoEventStoreDefinedError to ErrNoEventStoreDefined

Discussed this with @maxpersson in #8.
Merge pull request #7 from JensRantil/error-handling

Improved error handling