Change output errors as JSON when type set to json

[menghif]

Description

Fixes #141

This PR changes the output error to JSON when there is an error and the type is set to json:

{
"error": "User could not be found."
}

Type of change

• Bug fix (added a non-breaking change which fixes an issue)
• New feature (added a non-breaking change which adds functionality)
• Updated documentation (updated the readme, templates, or other repo files)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

• Tested locally with a valid username
• Tested locally with an invalid username
• Ran tests with composer test
• Added or updated test cases to test new features

Checklist:

• I have checked to make sure no other pull requests are open for this issue
• The code is properly formatted and is consistent with the existing code style
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

Screenshots

[DenverCoder1]

There is also another error card around line 24 of index.php

[DenverCoder1]

Works great! Thanks for contributing! 🎉

[menghif]

Hey, sorry it's my first time working with PHP but it's a great learning opportunity!
I just added the missing json error output. Let me know if there is anything missing.

[DenverCoder1]

It looks good. 👍 Usually I'd recommend putting any code that's repeated into a new function, but we've got #163 open that's taking care of that issue, so hopefully they will be able to incorporate this into their solution as well.