Stop build if vswhere.cmd can't properly set VC vars

[tylerszabo]

Description

Stop build if vswhere.cmd can't properly set VC vars

Motivation and Context

When attempting to build for the first time there were some issues that could result in a corrupted environment.

How Has This Been Tested?

• Attempted to configure for 32-bit then for 64-bit in the same CMD window.
• Attempted to repeat configuration 64-bit 6 times (PATH would be corrupt after ~3 before) via x64\make-mingwlibs.bat

Screenshots (if appropriate):

N/A

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

Checklist:

• My code follows the Code Guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the Contributing document
• I have added tests to cover my change
• All new and existing tests passed

[razzeee]

Thank you for this PR. I guess another review from @Rechi and this can be merged.