Fix exception message in cachi2.core.rooted_path.re_root

[slimreaper35]

The exception message in re_root currently states:
Joining path {s_other!r} to {s_self!r}: target is outside {s_root!r}

Proposed change (for example):
Update the message to a more user-friendly tone:
Path {s_self!r}/{s_other!r} outside {s_root!r}, refusing to proceed

Reason for change:
This rewording improves clarity and makes the message more direct and user-appealing.

A/C

• update the exception message
• fix unit tests
• remove asserts for the exact message capture

[eskultety]

Context: #752 (comment)

[eskultety]

Part of this effort should also be dropping all of the existing occurrences of pattern matching of the PathOutsideRoot exception string in the unit test suite.