VMR Change validation: provide sensible error message for bad source-mappings file

### Context

Currently, when the change validation pipeline runs on commits that have a badly formatted source-mappings.json file, it will return an error message that says "Unexpected error occurred".
This makes it harder to detect mistakes in the source-mappings file, and is arguably the reason why the following PR was merged with a typo, leading to PCS codeflow errors: https://github.com/dotnet/dotnet/pull/6044

### Goal

Failure to read the source-mappings.json file should lead to a clear, distinct error message propagating to azdo / github PRs. If possible, it should also contain information about what's wrong with the file
(example json error message: `'"' is invalid after a value. Expected either ',', '}', or ']'. Path: $.mappings[16] | LineNumber: 94 | BytePositionInLine: 12.`)