core: raise the log priority if sd-executor is missing

core: raise the log priority if sd-executor is missing

Log about missing executor at the emergency level, so the message always
makes it to the console - otherwise it won't get anywhere, since we
can't even start systemd-journald in that case.

Before:

Welcome to Arch Linux!

[    5.202479] systemd[1]: Failed to allocate manager object: No such file or directory
[!!!!!!] Failed to allocate manager object.
[    5.207741] systemd[1]: Freezing execution.

After:

Welcome to Arch Linux!

[    5.279408] systemd[1]: Failed to open executor binary '/usr/lib/systemd/systemd-executor': No such file or directory
[    5.290756] systemd[1]: Failed to allocate manager object: No such file or directory
[!!!!!!] Failed to allocate manager object.
[    5.295919] systemd[1]: Freezing execution.