Move error handling out of the network loader

[jdm]

There are a couple steps here:

• replace the free-form string in the Result that network listeners receive with the LoadError type from http_loader.rs
• propagate the errors from load_for_consumer to the network receiver instead of sending a string
• convert the other network loaders to use the LoadError values
• move the SSL validation error handling into the HTML parser network listener (servohtmlparser.rs)
• make headers_available take a Result<Metadata, LoadError> value to reflect the possibility of early errors (such as SSL validation failure)

[jdm]

This will both fix problems like <script src="http://some_invalid_ssl.com"> causing our error page to be interpreted as JS, and allow us to handle DNS failure properly in the HTML parser network listener (while other consumers will just see an early error).