iterate_file_lines() 

Add iterate_file_lines() to Genome::Sys.  This method is an abstraction to work through a file one line at a time.

Convert Genome::Model::Build::Command::DetermineError to use it.

There are probably other places it could be used.
Do you have an example where ```ERROR_FINDING_REGEX``` would do the wrong thing that I can add to the test?

Would adding the negative-look-behind for "called" to ``ERROR_FINDING_REGEX``` make it do the right thing (so find_die_or_warn_in_log() could go away), or does it really need to be a separate regex?