Added a bunch of FS utility functions for internal usage.
Changed the meaning of the input CA certificate file paths - now this are paths of directories with PEM files.
The base error structure is eventually extracted into a separate and 'context-neutral' unit.
Broad refactoring of SMTP writer's consumers is made to adapt code to a new meaning of PEM file paths.
Hide unnecessary custom errors.
Make minor changes over logger and parser code.
Merge branch 'pk_issue_6' into dev

Conflicts:
	internals_common_helpers_test.go
	internals_osfswrapper.go