Add a test that a sensible error is raised when MDBTools is not found

This is the error that happens in issue #23

tests/test_Arbin.py

@@ -3,6 +3,7 @@
 import os
 import sqlite3
 import subprocess
+import sys
 
 import pytest
 
@@ -13,6 +14,19 @@ from galvani import res2sqlite
 have_mdbtools = (subprocess.call(['which', 'mdb-export'], stdout=None) == 0)
 
 
+@pytest.mark.skipif(have_mdbtools, reason='This tests the failure when mdbtools is not installed')
+def test_convert_Arbin_no_mdbtools(testdata_dir, tmpdir):
+    """Checks that the conversion fails with an appropriate error message."""
+    res_file = os.path.join(testdata_dir, 'arbin1.res')
+    sqlite_file = os.path.join(str(tmpdir), 'arbin1.s3db')
+    if sys.version_info >= (3, 3):
+        expected_exception = FileNotFoundError
+    else:
+        expected_exception = OSError
+    with pytest.raises(expected_exception, match="No such file or directory: 'mdb-export'"):
+        res2sqlite.convert_arbin_to_sqlite(res_file, sqlite_file)
+
+
 @pytest.mark.skipif(not have_mdbtools, reason='Reading the Arbin file requires MDBTools')
 @pytest.mark.parametrize('basename', ['arbin1'])
 def test_convert_Arbin_to_sqlite(testdata_dir, tmpdir, basename):