[libcalamares] Give Python jobs a warning()

- Add libcalamares.utils.warning() alongside debug() for
   Python modules to output warnings.

src/libcalamares/PythonJob.cpp

@@ -99,6 +99,13 @@ BOOST_PYTHON_MODULE( libcalamares )
         bp::args( "s" ),
         "Writes the given string to the Calamares debug stream."
     );
+    bp::def(
+        "warning",
+        &CalamaresPython::warning,
+        bp::args( "s" ),
+        "Writes the given string to the Calamares warning stream."
+    );
+
     bp::def(
         "mount",
         &CalamaresPython::mount,

src/libcalamares/PythonJobApi.cpp

@@ -171,6 +171,11 @@ debug( const std::string& s )
     cDebug() << "[PYTHON JOB]: " << QString::fromStdString( s );
 }
 
+void
+warning( const std::string& s )
+{
+    cWarning() << "[PYTHON JOB]: " << QString::fromStdString( s );
+}
 
 PythonJobInterface::PythonJobInterface( Calamares::PythonJob* parent )
     : m_parent( parent )

src/libcalamares/PythonJobApi.h

@@ -66,6 +66,7 @@ boost::python::object gettext_path();
 boost::python::list gettext_languages();
 
 void debug( const std::string& s );
+void warning( const std::string& s );
 
 class PythonJobInterface
 {