burp: error: impl Display and Error

The `burp::error::Error` struct needs to implement the
`std::error::Error` trait (which requires that it also implement
`std::fmt::Display`) so that it can be used in dynamic dispatch
situations.  Notably, the `prometheus_exporter_base::render_prometheus`
function requires a closure that returns `Result<String, Box<dyn
std::error::Error>>`, so in order to use the `?` (early return)
operator on on the `metrics::get_metrics` function call, our error must
implement that trait.

src/burp/error.rs

@@ -1,4 +1,6 @@
 //! Client error handling
+use std::error;
+use std::fmt;
 use std::io;
 
 use serde_json;
@@ -40,3 +42,17 @@ impl From<serde_json::Error> for Error {
         Self::Protocol(error.to_string())
     }
 }
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            Self::Io(e) => write!(f, "I/O error: {}", e),
+            Self::Tls(e) => write!(f, "TLS handshake error: {}", e),
+            Self::Command(e) => write!(f, "Error from BURP server: {}", e),
+            Self::Protocol(e) => write!(f, "BURP protocol error: {}", e),
+            Self::UnsupportedVersion(e) => write!(f, "{}", e),
+        }
+    }
+}
+
+impl error::Error for Error {}