Add axum::Error (#150)

Replace `BoxStdError` and supports downcasting

src/error.rs

@@ -0,0 +1,28 @@
+use std::{error::Error as StdError, fmt};
+use tower::BoxError;
+
+/// Errors that can happen when using axum.
+#[derive(Debug)]
+pub struct Error {
+    inner: BoxError,
+}
+
+impl Error {
+    pub(crate) fn new(error: impl Into<BoxError>) -> Self {
+        Self {
+            inner: error.into(),
+        }
+    }
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.inner.fmt(f)
+    }
+}
+
+impl StdError for Error {
+    fn source(&self) -> Option<&(dyn StdError + 'static)> {
+        Some(&*self.inner)
+    }
+}