Detect cases when replacement tags are split due to different formatting

[centic9]

Currently, if the replacement-tags span multiple formats, replacement is simply not done, because we look at a run at a time. There are multiple options to handle this better:

• detect incorrectly formatted tags and fail gereation
• detect an unclosed tag and include text from the next run in this case

[centic9]

We now at least print out a warning. I did not want to stop processing as there might be cases where this is intended, i.e. where "${" is part of the document itself...