Implement Display and Error for ConfigError.

Closes #189.

lib/src/config/error.rs

@@ -1,6 +1,9 @@
 use std::path::PathBuf;
+use std::error::Error;
+use std::fmt;
 
 use super::Environment;
+use self::ConfigError::*;
 
 use term_painter::Color::White;
 use term_painter::ToStyle;
@@ -56,8 +59,6 @@ pub enum ConfigError {
 impl ConfigError {
     /// Prints this configuration error with Rocket formatting.
     pub fn pretty_print(&self) {
-        use self::ConfigError::*;
-
         let valid_envs = Environment::valid();
         match *self {
             BadCWD => error!("couldn't get current working directory"),
@@ -106,10 +107,47 @@ impl ConfigError {
     /// Whether this error is of `NotFound` variant.
     #[inline(always)]
     pub fn is_not_found(&self) -> bool {
-        use self::ConfigError::*;
         match *self {
             NotFound => true,
             _ => false
         }
     }
 }
+
+impl fmt::Display for ConfigError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match *self {
+            BadCWD => write!(f, "couldn't get current working directory"),
+            NotFound => write!(f, "config file was not found"),
+            IOError => write!(f, "I/O error while reading the config file"),
+            BadFilePath(ref p, _) => write!(f, "{:?} is not a valid config path", p),
+            BadEnv(ref e) => write!(f, "{:?} is not a valid `ROCKET_ENV` value", e),
+            ParseError(..) => write!(f, "the config file contains invalid TOML"),
+            BadEntry(ref e, _) => {
+                write!(f, "{:?} is not a valid `[environment]` entry", e)
+            }
+            BadType(ref n, e, a, _) => {
+                write!(f, "type mismatch for '{}'. expected {}, found {}", n, e, a)
+            }
+            BadEnvVal(ref k, ref v, _) => {
+                write!(f, "environment variable '{}={}' could not be parsed", k, v)
+            }
+        }
+    }
+}
+
+impl Error for ConfigError {
+    fn description(&self) -> &str {
+        match *self {
+            BadCWD => "the current working directory could not be determined",
+            NotFound => "config file was not found",
+            IOError => "there was an I/O error while reading the config file",
+            BadFilePath(..) => "the config file path is invalid",
+            BadEntry(..) => "an environment specified as `[environment]` is invalid",
+            BadEnv(..) => "the environment specified in `ROCKET_ENV` is invalid",
+            ParseError(..) => "the config file contains invalid TOML",
+            BadType(..) => "a key was specified with a value of the wrong type",
+            BadEnvVal(..) => "an environment variable could not be parsed",
+        }
+    }
+}