Harden `cds get` error handling for copy and path-resolution failures

[RonaldHensbergen]

Summary

cds get currently has a few failure paths where low-level exceptions can surface instead of a consistent GetError message. This can produce less actionable CLI output and inconsistent error handling.

Problem

• _add_copy_action can raise uncaught path/relative computation errors in edge cases.
• shutil.copy2(...) can raise uncaught OSError during file copy.
• Users should consistently receive clean ERROR ... output from cds get.

Proposed change

• Wrap path-resolution and copy operations in defensive error handling.
• Re-raise expected failures as GetError with context about source/destination.
• Preserve current success behavior while improving failure clarity and safety.

Acceptance criteria

• No uncaught traceback for expected filesystem/path failures in cds get.
• Errors are surfaced as GetError and reported via current CLI error flow.
• Existing cds get behavior remains unchanged on successful runs.