Allow `spack external find` (with no extra args) to proceed if the manifest file exists but
without sufficient permissions; in that case, print a warning. Also add a test for that behavior.
TODOs:
- [x] continue past any exception raised during manifest parsing as part of `spack external find`,
except for CTRL-C (and other errors that indicate immediate program termination)
- [x] Semi-unrelated but came up when discussing this with the user who reported this issue to
me: the manifest parser now accepts older schemas
See: https://github.com/spack/spack/issues/31191
f7ef064bdc