Add onlyOnError option

[sergejmueller]

Trigger a notification only on error.

[Turbo87]

This is not much different from the default behavior. Can you explain why you think that this should be included too?

[Gvozd]

This option different from current default behaviour, when build succeed(with no errors)
This option fully ignored alwaysNotify, excludeWarnings options, and lastBuildSucceeded
And only shows failed builds

Comparasion:

1. Success build, that fixed after previos error - are shown by default. But are not shown with onlyOnError
2. Warnings - are shown by default. But are not shown with onlyOnError

code may be simplification for the greater of clarity:

if (stats.hasErrors()) {
    error = findFirstDFS(stats.compilation, 'errors');
 } else if (this.options.onlyOnError) {
    return;
 } else if (stats.hasWarnings() && !this.options.excludeWarnings) {
... // current code

[Gvozd]

rebased manually