PCI/AER: Log actual error causes in aer_inject

The aer_inject driver is very quiet.  In most cases, it merely returns an
error code to user-space, leaving the user with little clue about the
actual reason for the failure.

So, log error messages for 4 of the most frequent causes of failure:
* Can't find the root port of the specified device.
* Device doesn't support AER.
* Root port doesn't support AER.
* AER device not found.

This gives the user a chance to understand why aer-inject failed.

Based on a preliminary patch by Thomas Renninger.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: Borislav Petkov <bp@suse.de>
CC: Thomas Renninger <trenn@suse.de>