Make project and interpreter lock acquisition non-fatal (#14404)
## Summary
If we fail to acquire a lock on an environment, uv shouldn't fail; we
should just warn. In some cases, users run uv with read-only permissions
for their projects, etc.
For now, I kept any locks acquired _in the cache_ as hard failures,
since we always need write-access to the cache.
Closes https://github.com/astral-sh/uv/issues/14411.
