More informative errors

[charlierudolph]

The errors thrown by aren't very useful. It'd be nice if they could be a bit smarter in specific cases.

Error thrown from a tag on a background

/home/bricas/site-tests/node_modules/cucumber/lib/cucumber/parser.js:21
          throw e;
          ^
CompositeParserException: Parser errors:
(4:3): expected: #TagLine, #ScenarioLine, #ScenarioOutlineLine, #Comment, #Empty, got 'Background: Override Page Load Timeout'
(5:5): expected: #TagLine, #ScenarioLine, #ScenarioOutlineLine, #Comment, #Empty, got 'Given I set pageLoadTimeout to 30000'
path: /home/bricas/site-tests/features/core/authentication-proxy.feature
    at ...

It'd be much nicer if we could say something like tags are not allowed on Background.

Error thrown for two features in a single file

/Users/charlesrudolph/github/cucumber-js/lib/cucumber/parser.js:21
          throw e;
          ^
CompositeParserException: Parser errors:
(53:1): expected: #EOF, #StepLine, #TagLine, #ScenarioLine, #ScenarioOutlineLine, #Comment, #Empty, got 'Feature: compilers2'
(54:3): expected: #EOF, #StepLine, #TagLine, #ScenarioLine, #ScenarioOutlineLine, #Comment, #Empty, got 'In order to use the JS dialect I'm most comfortable with'
(55:3): expected: #EOF, #StepLine, #TagLine, #ScenarioLine, #ScenarioOutlineLine, #Comment, #Empty, got 'As a step definition implementor'
(56:3): expected: #EOF, #StepLine, #TagLine, #ScenarioLine, #ScenarioOutlineLine, #Comment, #Empty, got 'I want to use any compiler to write my step definitions in'
(58:3): expected: #EOF, #StepLine, #TagLine, #ScenarioLine, #ScenarioOutlineLine, #Comment, #Empty, got 'Background:'
path: /Users/charlesrudolph/github/cucumber-js/features/compiler.feature
    at ...

it would be nice to say, cannot have multiple features in a single file

We won't be able to handle all errors but would be nice to give slowly build up move informative errors.