Fix handling of errors when fetching layers by URLs

[mtrmac]

gopls 0.15 identifies always-true/always-false conditions in the code; I broke that in fa54b28a4d7a7e97fe899f958326b2bd3f91a11a .

• Don’t ignore errors accessing layers by URLs. Note that this might break setups which were created in the past two years; they now don’t fall back to registry accesses.
• If we fail, also report all errors, not just the last one, as the original version of the code did.

[rhatdan]

LGTM @giuseppe PTAL

[giuseppe]

@mtrmac can you please rebase?

[mtrmac]

Rebased, tests pass.