tagpreview: open vim in readonly mode, and exit on failure (#1488)

- When computing the center line, 'exit' on failure instead of 'return'.
- Open vim in read-only mode to avoid a non-zero exit code if the file is
already opened.

Signed-off-by: Nicolas VINCENT <nico.vince@gmail.com>

bin/tagpreview.sh

@@ -30,10 +30,10 @@ else
   exit 1
 fi
 
-CENTER="$("${VIMNAME}" -i NONE -u NONE -e -m -s "${FILE}" \
+CENTER="$("${VIMNAME}" -R -i NONE -u NONE -e -m -s "${FILE}" \
               -c "set nomagic" \
               -c "${EXCMD}" \
-              -c 'let l=line(".") | new | put =l | print | qa!')" || return
+              -c 'let l=line(".") | new | put =l | print | qa!')" || exit
 
 START_LINE="$(( CENTER - FZF_PREVIEW_LINES / 2 ))"
 if (( START_LINE <= 0 )); then