Broken package metadata needs a better error message

Right now we just report the requirement that caused us to need the broken package. It would be nice to _actually_ point out the broken package and why.

Not sure yet what the right approach for this is. It will get more confusing in cases where we have more than one requirement _and_ one of them is broken.

See #182

Start by reading the context in #182 and tracing how the solver reports a requirement whose package metadata is broken. Define how the error should identify the broken package and explain the cause, including cases with multiple requirements; done means the resulting message distinguishes the broken package from the requirement that led to it.

Written by the indexing model from the issue text.