edac: Fix optional csrow ue_count collection (#3734)

PR #3720 introduced the node_edac_channel_uncorrectable_errors_total
metric.

It depends on the per-channel ue_count file (e.g. ch0_ue_count). This
isn't always available. Therefore, only log a debug message and keep the
existing collector working.

Signed-off-by: Christian Hoffmann <mail@hoffmann-christian.info>