theme: do not show not-found warning

Only show the "airline theme not found" warning message, when the user
actually used `:AirlineTheme foobar`, not when called by an autocommand
that tries to switch themes when the Vim colorscheme changed.

fixes #1824

plugin/airline.vim

@@ -151,7 +151,10 @@ endfunction
 
 function! s:airline_theme(...)
   if a:0
-    call airline#switch_theme(a:1)
+    try
+      call airline#switch_theme(a:1)
+    catch " discard error
+    endtry
   else
     echo g:airline_theme
   endif