Fix error message for missing configuration !includes

[GaryOkie]

Home Assistant release with the issue:!--
94.1 and probably all versions prior
-->

Last working Home Assistant release (if known):
n/a

Operating environment (Hass.io/Docker/Windows/etc.):

hassio raspi3 B+ 32GB sdcard

Component/platform:
core

Description of problem:

If an !include in configuration.yaml references a file that does not exist, the error message leads to chasing wild geese. The error states that config/configuration.yaml can't be found. It should instead state the name of the include file that can't be found.

In addition, consider a missing include file to be a warning level log message and just ignore it and continue processing configuration. That can be considered a FR, not essential.

hassio ha check
Error: Testing configuration at /config
Failed config
General Errors:

• File not found: /config/configuration.yaml
  Successful config (partial)

Problem-relevant configuration.yaml entries and (fill out even if it seems unimportant):
!include scripts.yaml

Traceback (if applicable):
nothing written to log file upon a restart since ha stops immediately, which is also distressing when attempting to diagnose the error.

Additional information:
See hassio post #120943 discussion