PR14927: move parser::print_error() to systemtap_session

PR14927: move parser::print_error() to systemtap_session

The parser::print_error() method was moved from the parser class to the
systemtap_session class in anticipation for colorization. The session
class previously already took care of error printing, except for parser
errors. Making all the error-printing methods part of the same class
will simplify the implementation of colorization.