Improve `Select` traces for deeply nested dependencies

[dschrempf]

Assume that we have a decl-use graph

A <- B <- C

That is, declaration A depends on declaration B, which in turn depends on declaration C.

Now, if we fail to parse declaration C, and select declaration A, we report something equivalent to

Could not select A because it depends on C which failed.

Instead, we should say:

Could not select A because it depends on B which depends on C which failed.