Grains cache ignores grain exceptions

Description of Issue

This issue is debatable, but I wasted some amount of time debugging a grain problem. In the end my custom grain was faulty, but grains_cache: True masked this fault.

What has happened:

1. A custom grain failed with an exception (and I haven't noticed this)
2. With grains_cache: True the grains were cached without this faulty grain
3. Subsequent grains.get calls returned an empty grain (because it wasn't cached) and the minion log was clear of any exceptions

What is expected:

If there were any exceptions when the grains were collected, the result should not be cached at all (i.e. the cache should be invalid and the next run should try to refresh the grains again). If any of the grains has failed, the error should be visible each time you run grains.get, even with caching enabled.

Steps to Reproduce Issue

1. Add a custom grain that raises any exception, sync it to the minion
2. Set grains_cache: True in a minion config
3. Run salt-call --local -l debug grains.get yourcustomgrain multiple times.
4. The exception will be visible only on the first run, then the grain is just empty and no errors is visible in the log

Versions Report

Happens at least on 2018.3.4 and 2019.2.0.

Reproduce the behavior with a custom grain that raises an exception, grains_cache: True, and repeated salt-call --local -l debug grains.get yourcustomgrain runs. Trace grain collection and cache handling from the grains.get entry point; done means failed collection results are not cached and the exception remains visible on subsequent calls.