log contentview errors in the debug log only
log contentview errors in the debug log only

We already display the "couldn't parse" message,
which is better than overwhelming users with a traceback.

refs #2658

mhils/mitmproxy,ujjwal96/mitmproxy,MatthewShao/mitmproxy,ujjwal96/mitmproxy,cortesi/mitmproxy,vhaupert/mitmproxy,ujjwal96/mitmproxy,MatthewShao/mitmproxy,mitmproxy/mitmproxy,zlorb/mitmproxy,cortesi/mitmproxy,Kriechi/mitmproxy,mitmproxy/mitmproxy,zlorb/mitmproxy,MatthewShao/mitmproxy,Kriechi/mitmproxy,mitmproxy/mitmproxy,Kriechi/mitmproxy,mhils/mitmproxy,vhaupert/mitmproxy,cortesi/mitmproxy,vhaupert/mitmproxy,ujjwal96/mitmproxy,mhils/mitmproxy,zlorb/mitmproxy,Kriechi/mitmproxy,MatthewShao/mitmproxy,mhils/mitmproxy,mitmproxy/mitmproxy,vhaupert/mitmproxy,mhils/mitmproxy,cortesi/mitmproxy,zlorb/mitmproxy,mitmproxy/mitmproxy