[MRG] Use find_executable for gs and raise error if not found (#166)

* Use find_executable for gs and raise error if not found

* Remove unused variable

* Add test

* Use pytest monkeypatch

tests/test_errors.py

@@ -60,3 +60,18 @@ def test_no_tables_found_warnings_suppressed():
         except Warning as e:
             warning_text = str(e)
             pytest.fail('Unexpected warning: {}'.format(warning_text))
+
+
+def test_ghostscript_not_found(monkeypatch):
+    import distutils
+
+    def _find_executable_patch(arg):
+        return ''
+
+    monkeypatch.setattr(distutils.spawn, 'find_executable', _find_executable_patch)
+
+    message = ('Please make sure that Ghostscript is installed and available'
+               ' on the PATH environment variable')
+    filename = os.path.join(testdir, 'foo.pdf')
+    with pytest.raises(Exception, message=message):
+        tables = camelot.read_pdf(filename)