Add test for exception with custom error

Also simplify the old test.

tests/test_exceptions.rs

@@ -1,6 +1,7 @@
 use pyo3::prelude::*;
-use pyo3::types::IntoPyDict;
-use pyo3::wrap_pyfunction;
+use pyo3::{exceptions, py_run, wrap_pyfunction, PyErr, PyResult};
+use std::error::Error;
+use std::fmt;
 use std::fs::File;
 
 mod common;
@@ -15,14 +16,61 @@ fn fail_to_open_file() -> PyResult<()> {
 fn test_filenotfounderror() {
     let gil = Python::acquire_gil();
     let py = gil.python();
-
     let fail_to_open_file = wrap_pyfunction!(fail_to_open_file)(py);
-    let d = [("fail_to_open_file", fail_to_open_file)].into_py_dict(py);
-    match py.run("fail_to_open_file()", None, Some(d)) {
-        Ok(()) => panic!("Call should raise a FileNotFoundError"),
-        Err(e) => {
-            py_assert!(py, e, "isinstance(e, FileNotFoundError)");
-            py_assert!(py, e, "'No such file or directory (os error 2)' == str(e)");
-        }
-    };
+
+    py_run!(
+        py,
+        fail_to_open_file,
+        r#"
+        try:
+            fail_to_open_file()
+        except FileNotFoundError as e:
+            assert str(e) == "No such file or directory (os error 2)"
+        "#
+    );
+}
+
+#[derive(Debug)]
+struct CustomError;
+
+impl Error for CustomError {}
+
+impl fmt::Display for CustomError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "Oh no!")
+    }
+}
+
+impl std::convert::From<CustomError> for PyErr {
+    fn from(err: CustomError) -> PyErr {
+        exceptions::OSError::py_err(err.to_string())
+    }
+}
+
+fn fail_with_custom_error() -> Result<(), CustomError> {
+    Err(CustomError)
+}
+
+#[pyfunction]
+fn call_fail_with_custom_error() -> PyResult<()> {
+    fail_with_custom_error()?;
+    Ok(())
+}
+
+#[test]
+fn test_custom_error() {
+    let gil = Python::acquire_gil();
+    let py = gil.python();
+    let call_fail_with_custom_error = wrap_pyfunction!(call_fail_with_custom_error)(py);
+
+    py_run!(
+        py,
+        call_fail_with_custom_error,
+        r#"
+        try:
+            call_fail_with_custom_error()
+        except OSError as e:
+            assert str(e) == "Oh no!"
+        "#
+    );
 }