Raise if conda-meta's pinned is ill-formed

[jakirkham]

Currently if someone adds content to conda-meta's pinned file that is ill-formed, it proceeds without raising or in the best case scenario may raise a tangential error message downstream. However this can really start to cause problems. For example, if one accidentally adds quotes around content in pinned that results in this content being blissfully ignored by Conda, causing surprising downstream consequences. Would be nice in these and other cases to validate pinned parses correctly and raise with an informative error if it doesn't. Thus blocking unintentional environment changes (which pinned is intended to avoid) and helping the user to diagnose what went wrong in pinned.

[github-actions]

Hi there, thank you for your contribution!

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Thanks!

[jakirkham]

Let's keep this open