GUI preview: improve operation when the index data is not up to date.

Avoid erasing all the file index data in case the subsequent update fails
(e.g. the file is locked). Improve the messages. Check for previous
indexing error, and modify the message.

.gitignore

@@ -20,9 +20,9 @@
 \#*
 libtool
 ptrans
-src/python/pychm/setup.py
 src/Makefile
 src/Makefile.in
+src/TAGS
 src/aclocal.m4
 src/autom4te.cache
 src/common/autoconfig.h
@@ -50,6 +50,7 @@ src/m4/ltsugar.m4
 src/m4/ltversion.m4
 src/m4/lt~obsolete.m4
 src/missing
+src/python/pychm/setup.py
 src/python/recoll/Makefile
 src/python/recoll/build
 src/python/recoll/recoll/__init__.pyc