Issue #778 - Group locale linter errors

This MR contains the following changes:

• Introduced ResultGroup class to represent a list of error strings, Error objects and ResultGroup objects that we can output as a single string.
• Return a ResultGroup object from each validate() function instead of throwing, in order to (a) collect all errors in a single result group and (b) run subsequent checks and not exit early. We should only skip subsequent checks, if they cannot be made (e.g. we cannot check a tag name, if we cannot find any tags).
• Moved string validation into file validation for more logical grouping.
• Updated tests.

The output for the example file from the issue description appears as:

[Success]: Test locale linter
[Fail]: Validate translations
    Validate '…/locale/en_US/foo.json'
        Validate strings
            Validate string 'foo-bar'
                [Error]: Invalid string ID 'foo-bar'
                Validate placeholders
                    [Error]: Unexpected placeholders
                Validate tags
                    [Error]: Unexpected tag name 'foo'