themes: correctly catch not-found in init function
themes: correctly catch not-found in init function

plugin/airline.vim was calling airline#switch_theme() without actually
catching the 'not-found' exception.

Therefore, in case of errors, explicitly switch to a dark theme.

closes #1903

bling/vim-airline,rchicoli/vim-airline,chrisbra/vim-airline,yzhou61/vim-airline