Raise helpful error for malformed output

I haven't actually observed this happening, but I want this engine to be
more assertive about its expectations.

lib/cc/engine/markdownlint.rb

@@ -7,6 +7,7 @@ module Engine
     class Markdownlint
       CONFIG_FILE = "./.mdlrc".freeze
       EXTENSIONS = %w[.markdown .md].freeze
+      UnexpectedOutputFormat = Class.new(StandardError)
 
       def initialize(root, engine_config, io, err_io)
         @root = root
@@ -51,8 +52,10 @@ def mdl_options
         options
       end
 
+      ISSUE_PATTERN = /(?<path>.*):(?<line_number>\d+): (?<code>MD\d+) (?<description>.*)/
+
       def issue(line)
-        match_data = line.match(/(?<path>.*):(?<line_number>\d+): (?<code>MD\d+) (?<description>.*)/)
+        match_data = line.match(ISSUE_PATTERN) or raise UnexpectedOutputFormat, line
         line_number = match_data[:line_number].to_i
         path = match_data[:path]
         relative_path = File.absolute_path(path).sub(root + "/", "")

spec/cc/engine/markdownlint_spec.rb

@@ -45,6 +45,17 @@ module Engine
           expect(issue["check_name"]).to eq("MD001")
         end
 
+        it "exits cleanly when the underlying tool outputs an unexpected format" do
+          io = StringIO.new
+          path = File.expand_path("../../fixtures/default", File.dirname(__FILE__))
+
+          child = double(out: "weird output", err: "")
+          expect(POSIX::Spawn::Child).to receive(:new).and_return(child)
+          expect {
+            CC::Engine::Markdownlint.new(path, {}, io, STDERR).run
+          }.to raise_error(CC::Engine::Markdownlint::UnexpectedOutputFormat)
+        end
+
         it "exits cleanly with empty include_paths" do
           io = StringIO.new
           path = File.expand_path("../../fixtures/default", File.dirname(__FILE__))