take advantage of the existing `From<T: ResponseError> for Error` in the implementation of `From<failure::Error>`.

This is better because it's re-using code, but also because my backtrace-handling was buggy.

src/error.rs

@@ -102,7 +102,7 @@ impl<T> ResponseError for failure::Compat<T>
 
 impl From<failure::Error> for Error {
     fn from(err: failure::Error) -> Error {
-        Error { cause: Box::new(err.compat()), backtrace: None }
+        err.compat().into()
     }
 }
 
@@ -662,11 +662,13 @@ pub fn ErrorInternalServerError<T>(err: T) -> InternalError<T> {
 
 #[cfg(test)]
 mod tests {
+    use std::env;
     use std::error::Error as StdError;
     use std::io;
     use httparse;
     use http::{StatusCode, Error as HttpError};
     use cookie::ParseError as CookieParseError;
+    use failure;
     use super::*;
 
     #[test]
@@ -795,4 +797,18 @@ mod tests {
         from!(httparse::Error::TooManyHeaders => ParseError::TooLarge);
         from!(httparse::Error::Version => ParseError::Version);
     }
+
+    #[test]
+    fn failure_error() {
+        const NAME: &str = "RUST_BACKTRACE";
+        let old_tb = env::var(NAME);
+        env::set_var(NAME, "0");
+        let error = failure::err_msg("Hello!");
+        let resp: Error = error.into();
+        assert_eq!(format!("{:?}", resp), "Compat { error: ErrorMessage { msg: \"Hello!\" } }\n\n");
+        match old_tb {
+            Ok(x) => env::set_var(NAME, x),
+            _ => env::remove_var(NAME),
+        }
+    }
 }