Avoid exception with lowercase doctype

[windymilla]

1. If there is a doctype error, e.g. doctype rubbish, output sensible error
2. Ignore case of doctype line of HTML file

Fixes #1939

[windymilla]

Test files: ppcomp_test.zip

In master this causes an exception
In this branch, I get the following - the HTML parser fails to parse the HTML file:

Failed to parse HTML file
Fix the errors below in the HTML file before re-running ppcomp

260.25: Tag nav invalid
321.87: Tag aside invalid
324.46: Tag section invalid
331.87: Tag aside invalid
333.87: Tag aside invalid
335.87: Tag aside invalid
337.87: Tag aside invalid
339.87: Tag aside invalid

Check complete

[srjfoo]

This setup doesn't appear to fail on macOS, but if the PR fixes it for Win/Linux, go for it.