Output embedded text in HTML report

The formatter in cucumber-html already supports embedding content
with mime-types starting with "text/" into the HTML report -
this just makes cucumber-jvm not swallow the text content before
it gets that far.
Improved error message when feature file has syntax errors
Fix for Lexing Error message not useful. Closes #519, #523
Merge branch 'html-formatter-embedded-text' of github.com:tomdcc/cucumber-jvm
Attribution. Closes #501.