Handle "invalid" input correctly#9

At the moment with `maud` v0.11.0 if a consumer of this library passes its markdown through a filter generating an unexpected series of events then `maud-pulldown-cmark` will exit early with an `Err` response, `maud` will just ignore the error and continue rendering the next part of the template. An example: ``` rust #![feature(plugin)] #![plugin(maud_macros)] extern crate maud; extern crate maud_pulldown_cmark; extern crate pulldown_cmark; use maud_pulldown_cmark::Markdown; use pulldown_cmark::{Event, Tag}; fn main() { let events = vec![ Event::Start(Tag::Image("http://example.com/image".into(), "title".into())), Event::Start(Tag::Code), Event::Text("hello".into()), Event::End(Tag::Code), Event::End(Tag::Image("http://example.com/image".into(), "title".into())), ]; let buffer = html!( div { (Markdown::from_events(events.into_iter())) } ); println!("{}", buffer.into_string()); } ``` saved to `examples/invalid.rs` and run with `cargo run --example invalid` gives: ``` <div><img src="http://example.com/image" alt="</div> ``` --- After the update to `maud` v0.12.0 in #8 the same example now gives: ``` <div><img src="http://example.com/image" alt="hello" title="title" /></div> ``` which is more valid, but still is silently dropping some of the input. Hopefully there is some way to report this sort of issue back to the user.