Function applyMarkerSection

Puts wrapped (content already between the start and end markers) into existing:

• no file, or an empty file: the new text is the file;
• one well-formed marker pair: only that section is replaced, the rest is kept byte for byte;
• no markers: throws MarkersMissingError unless allowAppend, which adds the section at the end;
• anything else (a lone marker, two sections, reversed markers): refused, nothing guessed.