Add error handling to Request methods

[brightly-salty]

Make all associated Request methods return a Result<Self, Error> instead of Self and panicking.
Change docs and doc-tests to reflect this, and remove manual parsing in doc-tests and unit tests

Fixes #303

[yoshuawuyts]

Few nits, but overall this is a really promising PR. Thank you!

[yoshuawuyts]

This does not need the map_err call to work:

Suggested change

	let url = url.try_into().map_err(|e| Error::new(500, e))?;
	let url = url.try_into()?;

But if it's for clarity purposes, it's preferable to use the Status trait instead:

Suggested change

	let url = url.try_into().map_err(|e| Error::new(500, e))?;
	let url = url.try_into().status(500)?;

[brightly-salty]

I got the following error after replacing map_err with status

no method named `status` found for enum `std::result::Result<url::Url, <U as std::convert::TryInto<url::Url>>::Error>` in the current scope

method not found in `std::result::Result<url::Url, <U as std::convert::TryInto<url::Url>>::Error>`

note: the method `status` exists but the following trait bounds were not satisfied:
      `<U as std::convert::TryInto<url::Url>>::Error: std::error::Error`
      which is required by `std::result::Result<url::Url, <U as std::convert::TryInto<url::Url>>::Error>: status::Status<url::Url, <U as std::convert::TryInto<url::Url>>::Error>`
      `<U as std::convert::TryInto<url::Url>>::Error: std::marker::Send`
      which is required by `std::result::Result<url::Url, <U as std::convert::TryInto<url::Url>>::Error>: status::Status<url::Url, <U as std::convert::TryInto<url::Url>>::Error>`
      `<U as std::convert::TryInto<url::Url>>::Error: std::marker::Sync`
      which is required by `std::result::Result<url::Url, <U as std::convert::TryInto<url::Url>>::Error>: status::Status<url::Url, <U as std::convert::TryInto<url::Url>>::Error>`