Fix error reporting in ParameterTreeParser and quoted values.

This should no longer just ignore lines that it does not understand.

It now supports comments after section headers.

THIS MAY BREAK SOME THINGS: The interaction between quoted values and comments is now sane, in the sense that you cannot use them on the same line. Previously, you could do things like

key = '# comment, not part of value'
not a # comment, actually part of value'

When a value starts with a quote, we will now scan until we find a line that ends with that quote and some optional whitespace. We will include any # characters literally in the value.

Apart from the insane example above, you can no longer do the somewhat sane things like

key = 'quoted value' # can't comment no more

On the plus side, you can now include # in the first line of your values:

docs = 'https://example.com/#documentation'

Closes: #54 (closed).