special exception for connection errors in libevent

[yannick]

ideas here:
a) make the errors catchable so it can be ignored or more silently logged
b) add some information about the peer if possible.

PR is probably not usable as is, but i want to discuss the Idea.

reason: i see lots of such errors in production (mainly from mobile ios devices):
(this is with below PR already included)

 vibe.core.drivers.utils.ConnectionException@../vibe.d/source/vibe/core/drivers/libevent2_tcp.d(432): Connection error while writing to TCPConnection. Remote: 127.0.0.1:55774
[EF29AF15:EF2B1315 2016.04.06 18:15:51.581 ERR] ----------------
[EF29AF15:EF2B1315 2016.04.06 18:15:51.581 ERR] ../vibe.d/source/vibe/core/drivers/libevent2_tcp.d:432 void vibe.core.drivers.libevent2_tcp.Libevent2TCPConnection.checkConnected(bool) [0x8e45bc]
[EF29AF15:EF2B1315 2016.04.06 18:15:51.581 ERR] ../v

[WebFreak001]

@s-ludwig no comment?

[s-ludwig]

Having proper separate exception types is definitely a pressing issue for the library in general. The problem is that it is going to be very difficult to switch from a custom exception hierarchy to exceptions defined in Phobos once they become available. However, if the depth of the hiearchy within vibe.d is kept to 1 and all those exception classes are marked final, it should be possible without breaking normal user code.

In this case, the question is if this should derive from std.socket.SocketException. Also, it should either be defined in a common place for vibe-d:core and vibe-core, or just in vibe-core so that no forward-compatibility issues get introduced with this.