Multiple Themes: Check for WP_Error before outputting `get_the_tag_list()`

[sixhours]

We've seen this error pop up on WordPress.com for multiple _s-based and default themes:

Catchable fatal error - Object of class WP_Error could not be converted to string

This happens because get_the_tag_list() can return an object of WP_Error and the theme doesn't account for it. Core handles it gracefully if the theme uses the_tags() instead.

Affected themes:

• Twenty Seventeen
• Twenty Fifteen
• Twenty Thirteen
• Twenty Twelve
• Twenty Eleven
• Twenty Ten

I propose we patch these to account for WP_Error. Two possible solutions:

1. If echoing the output, switch to the_tags().
2. If returning the tags list, add a catch for is_wp_error() in the function to avoid the fatal error.

I'll add some patches to get this started.

[sixhours]

Check for is_wp_error() before outputting get_the_tag_list() in Twenty Fifteen

[sixhours]

Check for is_wp_error() before outputting get_the_tag_list() in Twenty Seventeen

[sixhours]

Check for is_wp_error() before outputting get_the_tag_list() in Twenty Ten

[sixhours]

Twenty Sixteen is also affected; I opened a PR: ​https://github.com/WordPress/twentysixteen/pull/496

[davidakennedy]

Check for is_wp_error() before outputting get_the_tag_list().

[davidakennedy]

I combined the patches from @sixhours and moved the PR from Twenty Sixteen over.

See: ​https://github.com/WordPress/twentysixteen/pull/496

[ianbelanger]

This appears to have already been fixed. Closing

[SergeyBiryukov]

In 47886:

Bundled Themes: Consistently check for WP_Error before outputting get_the_tag_list().

Some themes already had these checks in place as a result of [41756], but not others.

Props sixhours, davidakennedy, SergeyBiryukov.
Fixes #39860.