Fix #915, Check for file existence in CFE_ES_RestartApp

[skliper]

Describe the contribution
Fix #915 - CFE_ES_RestartApp now checkes for file existence as part of command processing. Avoids removing the app if the file doesn't exist (just avoids one error case).

Testing performed
Build and ran unit tests, passed. Also ran cFE and tested with valid and invalid file (moved good file), confirmed error counter increment (and command rejected so app remained running).

Expected behavior changes
Now rejects command and increments command error counter if file is missing

System(s) tested on

• Hardware: cFS Dev Server
• OS: Ubuntu 18.04
• Versions: Bundle main + this commit

Additional context
#915

Third party code
None

Contributor Info - All information REQUIRED for consideration of pull request
Jacob Hageman - NASA/GSFC

[astrogeco]

CB:2021-01-27 APPROVED

• At least checks that the file exists
• Avoids unloading
• Follows the flow of restart and reload