Beautify Bazel configuration errors

**tl;dr:** apply [beautiful error](https://bazel.build/designs/2016/05/23/beautiful-error-messages.html) principles to build failures from analysis, `select()`, configurations, user-defined flags, transitions, and so on.

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We have enough experience to identify where current errors fall short. There are straightforward ways we make them clearer, more actionable, and more closely connected to relevant documentation.

I'm starting this for configuration-related errors since this is the Bazel area I know best. More broadly I'd like to extend these principles to Bazel error culture as a whole in the interest of making Bazel more accessible.

Crowd-sourced input on confusing messages welcome!

Start with the linked “beautiful error messages” design and inventory Bazel failures in analysis, select(), configurations, user-defined flags, and transitions. Identify representative confusing messages and define actionable, documentation-linked criteria for each; the work is done when the scope, owners, and validation approach are concrete.

Written by the indexing model from the issue text.