Bugfix: check target-file arg is non-nil before checking filename

Altered items:
org-dotemacs-load-file
Add code for error-handling and command-line args

New code to choose how org-dotemacs-load-blocks handles errors,
and variables for controlling behaviour from command line.

Altered items:
org-dotemacs-error-handling, org-dotemacs-tag-match,
org-dotemacs-load-file, org-dotemacs-load-blocks
Fix code for handling command line arguments

Also alter messages to begin with "org-dotemacs: " so they are easy to find in the
*Messages* buffer, and fix org-dotemacs-load-blocks to be defun* instead of just defun.

Altered items:

org-dotemacs-load-blocks