Alert users of misplaced tags in multi-output recipes

[cbouss]

In the context of multi-output recipes, placing sections at the wrong level leads to issues difficult to debug.
Especially, it happens that tags are used in the global build section instead of the build section of the output.
To make things more complicated, the script tag is an exception. When used on the output level, it has to be set at the same level as build instead of under build.

Ideally, we would follow a recipe schema.
We could check with conda if such a schema is available, and if not implement one based on conda-build source code.

[schuylermartin45]

Tracking: https://anaconda.atlassian.net/browse/PAT-174