Change "Bad" to "Invalid" in error names, mimicking io::Error::ErrorKind.
Change InvalidProxyCreds to ProxyUnauthorized.
Change DnsFailed to just Dns (the fact that there was a failure is implicit
in the fact that this was an error).
This adds a source field to keep track of upstream errors and allow
backtraces, plus a URL field to indicate what URL an error was
associated with.
The enum variants we used to use for Error are now part of a new
ErrorKind type. For convenience within ureq, ErrorKinds can be turned
into an Error with `.new()` or `.msg("some additional information")`.
Error acts as a builder, so additional information can be added after
initial construction. For instance, we return a DnsFailed error when
name resolution fails. When that error bubbles up to Request's
`do_call`, Request adds the URL.
Fixes#232.
