[packchk] no error message if ../etc/PACK.xsd not found

[jkrech]

Describe the bug
if packchk does not find the pack schema in the default location (../etc/PACK.xsd) to validate against no output is provided.

To Reproduce
Steps to reproduce the behavior:
run packchk without ../etc/PACK.xsd present

Expected behavior
A clear and concise error message should be provided that the PACK.xsd could not be found in <absolute path>.

*** ERROR M218:
  Unable to find schema file '<absolute path>/PACK.xsd' for validation

To be discussed:

• consider to continue the validation skipping the schema validation ?
• the tool header output is currently appearing after any schema check and there is not message indicating that in case the schema check was successful
• in case of schema errors this is the output:

packchk.exe ARM.CMSIS.pdsc --xsd c:\Keil_538a\UV4\PACK.xsd
Error at file C:/CMSIS-PACKS/ARM/CMSIS/5.9.0/ARM.CMSIS.pdsc, line 408, column 54: value 'ARM:81' not in enumeration

CMSIS Pack Checker 0.0.0+g9b4a2e2 (C) 2012-2023 Arm Ltd. and Contributors

Environment (please complete the following information):

• [packchk]
• Version [development branch]
• OS: [any]

Additional context
Add any other context about the problem here.

[thorstendb-ARM]

1. Message M218 has been changed to: "Unable to find schema file for validation: '%PATH%'"
   In case of default location of PACK.xsd, %PATH% iss expanded to "./, ../etc/, ../../etc/" (which is where packchk searches).
2. consider to continue the validation skipping the schema validation ?
   yes
3. the tool header output is currently appearing after any schema check and there is not message indicating that in case the schema check was successful
   fixed

[thorstendb-ARM]

https://github.com/Open-CMSIS-Pack/devtools/releases/tag/tools%2Fpackchk%2F1.4.0

[jkrech]

Validating version 1.4.0:

packchk.exe ARM.CMSIS-NN.pdsc


*** ERROR M218:
  Unable to find schema file for validation: './, ../etc/, ../../etc/'

*** ERROR M105:
  Error while parsing command arguments

[thorstendb-ARM]

Reviewed messages:

CMSIS Pack Checker 0.0.0+g9471c31 (C) 2012-2023 Arm Ltd. and Contributors

*** ERROR M218:
  Unable to find PACK.xsd schema file. Searched in './, ../etc/, ../../etc/' relative to 'D:\GIT\devtools-external\build\tools\packchk\windows-amd64\Debug\'.
  Alternatively use --xsd to specify the location of the schema file. 

Log created on Fri Jun  2 10:26:01 2023

CMSIS Pack Checker 0.0.0+g9471c31 (C) 2012-2023 Arm Ltd. and Contributors

*** ERROR M219:
  Unable to find specified schema file in 'D:/GIT/devtools-external/build/tools/packchk/foo.xsd'

Log created on Fri Jun  2 10:31:18 2023

see https://github.com/Arm-Debug/devtools-external/pull/590