Add error message if no suitable image viewer found

uoeia

@@ -5,27 +5,27 @@ show_usage() {
 
 Usage: uoeia [OPTION]... [FILE|URL]
 
-Displays the specified image using the nsxiv program, or an image viewer of your choice.
+Displays the specified image using your preferred image viewer.
 
 Can open remote urls of individual images, galleries, as well as local files at the same time.
 Uses gallery-dl for remote gallery opening - so if gallery-dl can open it, so can your image viewer.
 
 Options:
-  -v, --viewer       Specify the image viewer to use (default: nsxiv).
+  -v, --viewer       Specify the image viewer to use.
   -q, --quiet        Don't display any output during normal operation.
   -g, --gdlopts      Pass through options to gallery-dl. Take care to fully quote each passed option.
   -r, --replace      Url patterns to replace in the format 'mypattern:::myreplacement'
   -h, --help         Show this help message and exit.
 
 Examples:
-  uoeia my-image.jpg                        Display a local image file using nsxiv.
+  uoeia my-image.jpg                        Display a local image file.
 
-  uoeia https://example.com/image           Display an image/image gallery from a URL using nsxiv.
+  uoeia https://example.com/image           Display an image/image gallery from a URL.
 
   uoeia -v imv https://example.com/image    Display an image from a URL using imv.
 
   uoeia -g \"--range 1-10\" https://reddit.com/r/earthporn
-                                            Download and display the first 10 results using nsxiv.
+                                            Download and display the first 10 results.
 
   uoeia -r 'nitter.net:::twitter.com' https://nitter.net/donttrythis/
                                             Display images from twitter directly instead of its
@@ -33,9 +33,6 @@ Examples:
 "
 }
 
-cache_dir="$(mktemp -d)"
-imageviewer=nsxiv
-
 die() {
     [ -n "$1" ] && printf '%s\n' "$*" >&2
     exit 1
@@ -88,11 +85,19 @@ dereference() {
     echo "$output"
 }
 
+get_imageviewer() {
+    for cmd in nsxiv sxiv feh imv vimiv; do
+        if command -v "$cmd" 1>/dev/null 2>&1; then program="$cmd"; break; fi
+    done
+    echo "$program"
+}
+
 main() {
-    command -v "${imageviewer}" 1>/dev/null 2>&1 || die "Opening images in ${imageviewer} requires ${imageviewer} to be installed and and on the path."
+    [ -n "$imageviewer" ] || die "Error: Could not find a suitable image viewer, set one manually with the '-v' option."
+    command -v "${imageviewer}" 1>/dev/null 2>&1 || die "Error: Opening images in ${imageviewer} requires ${imageviewer} to be installed and and on the path."
 
     trap cleanup EXIT
-    [ -d "$cache_dir" ] || mkdir -p -- "$cache_dir" || die
+    [ -d "$cache_dir" ] || mkdir -p -- "$cache_dir" || die "Error: Can not create cache directory {$cache_dir}"
     im_number=1
     while [ -n "$1" ]; do
         case "$1" in
@@ -107,8 +112,11 @@ main() {
         ${imageviewer} "$cache_dir"
 }
 
+cache_dir="$(mktemp -d)"
+imageviewer="$(get_imageviewer)"
 gal_args=()
 replacements=()
+
 while [[ $# -gt 0 ]]; do
     case "$1" in
     -h | --help)