Async cache won't store exceptions

[akmalviya03]

Earlier async cache was caching exceptions.
With new changes in fetch method of AsyncCache class will no longer store exception.
By default AsyncCache will store exception, by setting _canCacheException to false. It won't store exception.

Please refer #257 for video demo of both issue and solution along with minimal reproducible code.

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[MDSADABWASIM]

Any update on this @akmalviya03 @lrhn

[lrhn]

SGTM.
@natebosch WDYT?

[natebosch]

@natebosch WDYT?

This one doesn't have the same issue with a Future field that loses it's expected association like #260

I don't think I have any concerns about landing this one.

They are interesting to consider together though - adding this capability might give stronger motivation for accepting #260

[akmalviya03]

@natebosch @lrhn Any update on this ?