Channel alert pipe: improve diagnostic error return

When a frame is queued on a channel, any failure in
ast_channel_alert_write is logged along with errno.

This change improves the diagnostic message through
aligning the errno value with actual failure cases.

ASTERISK-25224
Reported by: Andrey Biglari

Change-Id: I1bf7b3337ad392789a9f02c650589cd065d20b5b