More visible error on broken inputs

[jaraco]

Currently, pip-run is very lenient about reading dependencies. It does so because it aims to be generous about what inputs it reads for requirements, so wants to ignore invalid ones.

However, as projects like coherent.build start using this functionality for the build backend metadata discovery, silently ignoring subtle errors makes it difficult to troubleshoot. Take coherent-oss/coherent.build#17 as an example.

Probably, the dependency reader should suppress select errors and conditions that are known to be ignorable, but allow others to bubble up.