Uncaught crash when given job does not exist in workflow yaml

[coderobe]

currently dies like this

Traceback (most recent call last):
        4: from /usr/local/bin/miniexec:23:in `<main>'
        3: from /usr/local/bin/miniexec:23:in `load'
        2: from /var/lib/gems/2.7.0/gems/miniexec-0.2.7/bin/miniexec:58:in `<top (required)>'
        1: from /var/lib/gems/2.7.0/gems/miniexec-0.2.7/bin/miniexec:58:in `new'
/var/lib/gems/2.7.0/gems/miniexec-0.2.7/lib/miniexec.rb:42:in `initialize': undefined method `[]=' for nil:NilClass (NoMethodError)

should maybe throw a human readable error along the lines of $job is not defined in $workflow.yml