rootless: Tell the user what was led to the error, not just what it is

Users coming e.g. from Docker do not always read the manual and
expect podman to not require sudo or uidmap, for them the default
message is not very helpful:

    Error: Cannot connect to the Podman socket, make sure there is a Podman REST API service running.:
    cannot find newuidmap: exec: "newuidmap": executable file not found in $PATH

Adding a bit more context to this would help to nudge them into the
right direction and tell them what to look for in the documentation:

    command required for rootless mode with multiple IDs: exec: "newuidmap": executable file not found in $PATH

Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk>

[NO TESTS NEEDED]

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