Improve json schema and schema errors

[theotherjimmy]

Resolves #6284

[LiyouZhou]

tested locally, resolves the issue reported.

[theotherjimmy]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1364
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6285/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1016
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6285/

[mbed-ci]

Test : FAILURE

Build number : 1144
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6285/1144

[cmonr]

ಠ_ಠ
/morph test

[mbed-ci]

Test : SUCCESS

Build number : 1148
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6285/1148