Print out details of AggregateError originating from Spectral (@microsoft.azure/openapi-validator v2.1.4 -> v2.1.5)

[konrad-jamrozik]

Fixes:

• https://github.com/Azure/azure-sdk-tools/issues/6856

See this comment for details.

Deployment information: https://devdiv.visualstudio.com/DevDiv/_git/openapi-alps/pullrequest/499394