We want to process errors for individual subscribers.
Subscriber errors were inlined into error message string.
This commit changes this so that we are now able to get
subscriber errors as a data which are easy to process further.
[MAILPOET-1154]
We want to add some logic to error handling.
This commit extracts error handling code from sending methods classes,
which already do a lot of other stuff, to error mappers which are responsible
for creating proper error object (MailerError). This error object is a replacement
for assoc. array which already had some special keys for certain usecases and
can not be properly type hinted.
[MAILPOET-1154]
