Move TryFromError from generated code into main crate

There is not really a point in generating this type for every
invocation. Also reduces the number of things magically generated.

src/lib.rs

@@ -2,3 +2,32 @@
 pub use confique_macro::config as config;
 
 pub use serde;
+
+
+use std::fmt;
+
+
+/// Error for the `TryFrom` conversion from raw types to the main types.
+///
+/// This error is returned when a required value is `None` in the raw type.
+#[derive(Clone)]
+pub struct TryFromError {
+    /// This is only public so that macro generated code can created instances
+    /// of this type.
+    #[doc(hidden)]
+    pub path: &'static str,
+}
+
+impl fmt::Display for TryFromError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        std::write!(f, "required configuration value is missing: '{}'", self.path)
+    }
+}
+
+impl fmt::Debug for TryFromError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::Display::fmt(self, f)
+    }
+}
+
+impl std::error::Error for TryFromError {}