CLN: let URLError fall through in pandas.io.html._read
cc @jtratner
#4303 
 @cpcloud handling network IO is clearly tricky. Were you able to recreate the conditions for the error that was initially raised with the original URL?
 nope. just skipping for now if raising `IOError` (this is what the `network` decorator does), but there's something going on with a bogus url that i'm passing in tests with unicode not sure what yet
 marking for 0.13 because i'm not sure if i'll be able to get this by this weekend
 oh phooey i put in 0.12 release notes :expressionless: 
