Support identification of _meta-related issues

While handling `_meta`, we tried to be very conservative, defaulting to NumPy anywhere we couldn't find an appropriate solution. We have a few ways of handling such situations, including maintaining the current behavior, as noted by @mrocklin in https://github.com/dask/dask/pull/4914#issuecomment-502111112.

Option 2 seems to be a good compromise between usability and developer support, allowing developers to more quickly identify issues that cause Dask to default to NumPy.

Such a warning should then be added to several parts of Dask, as for example `blockwise` and `meta_from_array` functions. Warnings suppressed by #5103 should be reenabled.