K2V client improvements (#307)

- [x] Better distinguish error types
- [x] Parse error messages received from server
- [x] Remove `src/` folder layer, we don't have that for other crates

Co-authored-by: Alex Auvolat <alex@adnab.me>
Reviewed-on: https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/307
Co-authored-by: Alex <alex@adnab.me>
Co-committed-by: Alex <alex@adnab.me>

src/k2v-client/Cargo.toml

@@ -6,6 +6,7 @@ edition = "2018"
 [dependencies]
 base64 = "0.13.0"
 http = "0.2.6"
+log = "0.4"
 rusoto_core = "0.48.0"
 rusoto_credential = "0.48.0"
 rusoto_signature = "0.48.0"
@@ -22,6 +23,10 @@ garage_util = { path = "../util", optional = true }
 [features]
 cli = ["clap", "tokio/fs", "tokio/io-std", "garage_util"]
 
+[lib]
+path = "lib.rs"
+
 [[bin]]
 name = "k2v-cli"
+path = "bin/k2v-cli.rs"
 required-features = ["cli"]