Fix English in exception message

Change-Id: If0a4ed137ee13efd2714eb3f5e1a1ba6d1f39c9a
[FIXED JENKINS-15966] if a build cannot be scheduled show the reason why not a null pointer/method error.

The Exception was missing the constructor that was called in the case where we failed to schedule a job.
Added the missing constructor to the exception and added a test case.

The failure reason highlights the need for some higher managment for flows to handle blocking on downstream jobs
and not exhausting the exectutors.  This will be addressed in a extension plugin.

Change-Id: I7aaca30bc86501a44c1e0e126525be54db4a372a
Fixed typo when a job could not be found.

Added testcase for the message, and added test case for running a disabled job.

Change-Id: I94329089b03f665bad0d263bcd769596f676fd04
Allow extensions to also register DSL help.

An extension provides help by providing help-dsl.jelly or help-dsl.html under a folder
corresponding to the extension classname.

Change-Id: Iae6b572819ff8a8f7e7d4f424d573a8849aa3776
[FIXED JENKINS-19423] Aborting a buildflow should abort/cancel any jobs started/scheduled by the flow.

If a user aborts a flow via the UI the abort is propagated to all jobs that the flow has queued.
If the job is queued, it is cancelled, if it is started then it is aborted with an appropriate cause.

Change-Id: I7b7e4a89c817138de437a94007554d67a76cbe29
Various UI tweaks.

1) Add a section for the build definition so it doesn;t merge with the triggers
2) Add links for the abort cause back to the flow
3) add links for the trigger cause back to the flow

Change-Id: I7923e1d9afea54363a904de0acb85a100b484b97
Merge pull request #35 from jtnord/scheduling

scheduling / abort and UI tweaks