Nicer error message when activating nonexistent rule

[apjanke]

Currently, if I try to activate a non-existent rule in a style file, I get a hard-to-read stack trace error.

rule 'bogus-rule'

$ mdl _pages --style _linter/markdown-linter-style.rb                              ✘ 127 master ✱
Traceback (most recent call last):
	7: from /Users/janke/bin-local/mdl:23:in `<main>'
	6: from /Users/janke/bin-local/mdl:23:in `load'
	5: from /usr/local/lib/ruby/gems/2.5.0/gems/mdl-0.5.0/bin/mdl:10:in `<top (required)>'
	4: from /usr/local/lib/ruby/gems/2.5.0/gems/mdl-0.5.0/lib/mdl.rb:26:in `run'
	3: from /usr/local/lib/ruby/gems/2.5.0/gems/mdl-0.5.0/lib/mdl/style.rb:51:in `load'
	2: from /usr/local/lib/ruby/gems/2.5.0/gems/mdl-0.5.0/lib/mdl/style.rb:51:in `instance_eval'
	1: from _linter/markdown-linter-style.rb:4:in `load'
/usr/local/lib/ruby/gems/2.5.0/gems/mdl-0.5.0/lib/mdl/style.rb:30:in `rule': undefined method `params' for nil:NilClass (NoMethodError)

This PR adds an input check to produce a nicer error message when referencing a non-existent rule.

It also checks to see if rule was passed a block, which probably means that someone called the style rule DSL, when they meant to call the ruleset style DSL.

Would help with #244.

[dgattey]

Nice - this is clean! Hope it gets merged

[jaymzh]

I pushed a commit to this branch do switch to unless and will merge.

[apjanke]

👍