Warn if template specified by "layout" in front matter does not exist

[lpar]

Followup to discussion on forum.

If files include a layout = value in their frontmatter, and there is no matching template, they are silently ignored.

Instead, some sort of error should be generated, or at least a warning. Something along the lines of:

No template matching layout 'foo' for file/name/here.md

This would have saved me quite a bit of debugging time.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. The resources of the Hugo team are limited, and so we are asking for your help.
If this is a bug and you can still reproduce this error on the master branch, please reply with all of the information you have about it in order to keep the issue open.
If this is a feature request, and you feel that it is still relevant and valuable, please tell us why.
This issue will automatically be closed in the near future if no further activity occurs. Thank you for all your contributions.

[vassudanagunta]

#3514 would be a good place for this.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. The resources of the Hugo team are limited, and so we are asking for your help.
If this is a bug and you can still reproduce this error on the master branch, please reply with all of the information you have about it in order to keep the issue open.
If this is a feature request, and you feel that it is still relevant and valuable, please tell us why.
This issue will automatically be closed in the near future if no further activity occurs. Thank you for all your contributions.

[kaushalmodi]

I think solving this issue is critical for better site debug. Please don't let this just drop off.

[bep]

I think this would be more useful in this area: #4481 -- I'm not totally sure about this one.

[kaushalmodi]

Solution to that issue would be even better! May be that trace shows a warning that as the layout specified in front matter does not exist, Hugo is using the default available as per lookup order.

[jmooring]

I'm not sure when this changed, but we do not "silently drop content" if the layout specified in front matter does not exist. Instead, we fallback to Hugo's defined lookup order.