Idiomatic rust organizes unit tests into `mod tests { }` blocks
using conditional compilation `[cfg(test)]` to decide whether to
compile the code in that block.
This commit moves "bare" test functions into such blocks, and puts
the block at the bottom of respective file.
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.
