Document Error and its constructor

I'm assuming `Error.this` should be documented, and have chosen the one without `file` and `line` parameters as they don't have default arguments.

* `bypassedException` already had a doc-comment, but we should perhaps check it's really ready to be included in the docs as `Error` itself was undocumented.
* BTW should `bypassedException` be an `Exception`, not `Throwable`?

We could un-doc-comment `bypassedException` for now if unsure. Copying @donc who added it in 0101752c.