verilator: fix src_type defaulting to 'C'

After the sections refactoring, verilator lost its
ability to default src_type to 'C'. This patch brings
back this feature.

It also changes the associated error message syntax.

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>
icarus.py: Use Launcher instead of calling subprocess directly
modelsim.py: Clean up and remove exit(1)
remove utils/launch and replace launch with Laucher in quartus.py
add module sections.py to Makefile.am