Log: libcrmcommon: Warn instead of err in scan_ll()

Logging at err level in the event of a parse failure makes scan_ll() too
chatty. Specifically, it causes crm_node -q/-n to always log a parse
failure to stderr when run within a bundle, where logging to stderr is
enabled by default via PCMK_stderr.

In the future we may want to drop to crm_debug() in scan_ll(). This will
require looking for particular callers where more visible error checking
is necessary, and logging at higher levels there.

Resolves: RHBZ#1874391

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>