Add help text to explain why hooks do not run

[timClicks]

This PR provides some more context to unassuming new developers who are learning about BeforeHooks/AfterHooks.

Using check's comment feature, this message is displayed:

// At least one hook has not run. Some common causes:
    // - forgetting to prefix the call to Check() with defer
    // - checks in the build function failing, so the slice of txn.Ops never gets run
    // - miscalculating the number of hooks and/or attempts
    c.Assert(remaining, gc.HasLen, 0)
	... obtained []txn.TestHook = []txn.TestHook{txn.TestHook{Before:(func())(0x2f78a10), After:(func())(0x2f78bb0)}, txn.TestHook{Before:(func())(0x2f78a10), After:(func())(0x2f78bb0)}, txn.TestHook{Before:(func())(0x2f78a10), After:(func())(0x2f78bb0)}}
... n int = 0

Hopefully this is more useful to newcomers than the bald Assert failure:

c.Assert(remaining, gc.HasLen, 0)
... obtained []txn.TestHook = []txn.TestHook{txn.TestHook{Before:(func())(0x2f78a10), After:(func())(0x2f78bb0)}, txn.TestHook{Before:(func())(0x2f78a10), After:(func())(0x2f78bb0)}, txn.TestHook{Before:(func())(0x2f78a10), After:(func())(0x2f78bb0)}}
... n int = 0

[babbageclunk]

Looks great, assuming the comment gets displayed nicely in the output!

[babbageclunk]

Maybe this wording for point 2?

- checks in the build function failing, so the slice of txn.Ops never gets run

Just to make it more concrete where to look for the problem.

[timClicks]

Done

[timClicks]

$$merge$$

[babbageclunk]

!!build!!

[babbageclunk]

$$merge$$