Allow json errors for this test

..because it on-purpose throws locking errors
Uniformly shape catch( Throwable )

Allows catching IllegalArgumentException, which is tossed by the locking
code now when you cannot get exclusive access.  This lets the GUI report
nicely that you cannot e.g. delete a key currently being used by
model-building
Lock in parser BEFORE going background

Also allow upgrade from read-lock to write-lock.
Also allow delete of write-locked keys.
Parser read-locks inputs.
Allow upgradable locks

Can acquire write-lock if unlocked or sole read-locked owner.
Can delete if acquire write-lock (after upgrade).
Fix some internal lock asserts
Merge branch 'master' of https://github.com/0xdata/h2o