Enrich gssapi::Error with message from libgssapi::error::Error (#39)

```
GSS-API error: Miscellaneous failure (see text) (Error from KDC: UNKNOWN_SERVER while looking up 'zookeeper2/[email protected]' (cached result, timeout in 1184 sec) (negative cache))
```

Comparing to simple `GSS-API error`.

Author: dequbed

src/mechanisms/gssapi/properties.rs

@@ -5,7 +5,7 @@ use thiserror::Error;
 
 #[derive(Debug, Error)]
 pub enum Error {
-    #[error("GSS-API error")]
+    #[error("GSS-API error: {0}")]
     Gss(
         #[source]
         #[from]