This is optional but now enabled for release builds.
Why?
* It shrinks the release docker images a bit, as the binary
includes only the gzipped version of files and uncompressed into RAM
at startup (which should be fast).
* It's a step toward #160.
HTTP requests were only returning the error message to the caller, not
logging locally. In most cases the problem could be understood
client-side, but there are some exceptions. E.g. if Moonfire returns
a 403 on WebSocket update, even in the Chrome debug tools's network
tab the HTTP response body seems to be unavailable. And in general,
it's nice to have more context server-side.
Logging a `response::Body` isn't practical (it could be a stream), so
convert all the web stuff to use `base::Error` err returns.
Convert the `METHOD_NOT_ALLOWED` paths to return `Ok` for now. This is a
bit lame but punts on having some way of plumbing an explicit/overridden
status code in `base::Error`, as no gRPC error kind cleanly maps to
that.
Also convert `db::auth`, rather than making up an error kind in the web
layer.
This is also a small step toward getting rid of `failure::Error`.
