Store hash before decoding, do not parse broken file twice

[ties]

Tiny tweak. My previous interpretation (but have not tested it) is that a SLURM or JSON file that fails to parse will be parsed every time.

• log when slurm file is updated
• prevent parsing of file which we already know is broken

Trivial change. Prevents a series of "updating from x to y" when y is in fact not parseable.

[ties]

Tested locally. Inconclusive results because -checktime is broken and I have static test files.