Better error when an invalid value is provided for the --json flag

[shreyas-goenka]

Changes

I found myself making this mistake multiple times where I forget to include @ before the path and the error returned is confusing. This PR improves that bit of UX.

Before:

➜  cli git:(better-json-error) databricks jobs create --json .codegen.json
Error: invalid character '.' looking for beginning of value

After:

➜  cli git:(better-json-error) cli jobs create --json .codegen.json
Error: failed to parse JSON string. Please ensure that the value provided to the --json flag is either a valid JSON string or @path/to/file.json with valid JSON content: invalid character '.' looking for beginning of value

Tests

manually

[andrewnester]

I'd prefer it to be built on top of this PR because it changes a lot how we load JSON input and new error message likely belong to new json loader

#1771