REFACTOR: Correctly use parse errors as a cause.

src/build/mod.rs

@@ -329,11 +329,17 @@ impl<'a> FileBuilder<'a> {
                     Some(val) => Ok(val),
                 }
             }
-            Err(err) => Err(Box::new(error::BuildError::with_pos(
-                format!("{}", err,),
-                error::ErrorType::ParseError,
-                (&input).into(),
-            ))),
+            // TODO(jwall): We can probably use actual errors now?
+            Err(err) => {
+                let cause = Box::new(simple_error::SimpleError::new(err));
+                Err(error::BuildError::with_pos(
+                    "Unable to parse input",
+                    error::ErrorType::ParseError,
+                    (&input).into(),
+                )
+                .wrap_cause(cause)
+                .to_boxed())
+            }
         }
     }