Giving and taking is what is necessary for everyone to complete a
safe journey. It takes everyone to work together, to give way when
at minor road or to maintain right of way at major road. At all
times, being cautious as well. Seems familiar right?
When inside the roundabout, to stay in the roundabout, keep
signaling right. To exit to the upcoming road, signal left in
sufficient time.
As to your situation, IMO, I feel it is no one fault if you signal
right to let others know your intent not to exit anytime
soon.
The other car on the other hand should know when he need to exit
and signal with sufficient indication time. He also should slow
down safely to a reasonable speed to you, him and others to react,
and try to filter out by letting you go first, if you were ahead.
The car behind you should give way to the other car if reasonably
possible and if safe to other traffic to do so.
If he did not realise until last moment, the safe and considerate
way to handle it is to continue another round, safely filtering
left and signalling intent while filtering. While not exiting in
the roundabout, should still signal right until a reasonable
distance from the desired exit.
As he horned you, well, could be the following few reasons:
1) depressed the horn accidently
2) he wanted to exit, got excited and quickly wanted to filter out
by horning you the intent. Who dun horn when excited. (For me so
many times, car behind me cannot see my front and do not know got
slow vehicle or bicycle or people, then horn me to go faster. Well,
got to bear it and ignore them and maintain slow speed. When path
clear, then accelerate carefully. Never fear the car behind, be
careful of the traffic ahead. But of course cannot jam brakes too
lah. Slow down sufficiently before traffic lights, if not either
kenna red light camera or car back kenna crash.)