[ADDED] ReconnectErrCB + handler function

[sschleemilch]

Hi,

I am often using those connect options due to an offline first approach:

opts := []nats.Option{
    nats.RetryOnFailedConnect(true),
    nats.MaxReconnects(-1),
}

I noticed that none of the current handlers covers handling connection attempt errors when there never was an initial connection with those options.
This is probably a very narrow use case.

That is why I thought adding ReconnectErrCB and the corresponding ReconnectErrHandler makes sense.

Since this is my first contribution, please let me know whether I missed something or whether it should be solved
in a complete different manner. I am willing to put in more work if necessary of course.

Sorry for having a bit of additional diffs due to using gopls formatting. I did not find any formatting rules.
If there are any, I am happy to reformat accordingly.

Regards

[coveralls]

coverage: 84.846% (-0.08%) from 84.922%
when pulling 7a85aa5 on sschleemilch:main
into 7885c8c on nats-io:main.