Add helpful messages if missing requirements

OCR/image-alignment/go.sh

@@ -1,9 +1,26 @@
 #!/bin/bash
-# download pertussis PDFs from:
-# https://drive.google.com/drive/folders/1vG-XwFiOoRunx9eXCmd41WpAI5NCizeM
 
 set -eu
 
+HELPSTR="
+Download these files:
+  0_Pertussis.fillable_Redacted.pdf
+  1_Pertussis.scanned paper_Redacted.pdf
+
+from this Google Drive folder:
+  https://drive.google.com/drive/folders/1vG-XwFiOoRunx9eXCmd41WpAI5NCizeM
+"
+
+if [[ ! -f "0_Pertussis.fillable_Redacted.pdf" && ! -f "1_Pertussis.scanned paper_Redacted.pdf" ]]; then
+    echo "$HELPSTR"
+    exit 1
+fi
+
+if [[ ! -x $(which convert) ]]; then
+    echo "This script requires ImageMagick and the \`convert\` binary to be available."
+    exit 1
+fi
+
 # Extract the first page of each PDF and rasterize it as a PNG
 convert -density 300 "0_Pertussis.fillable_Redacted.pdf[0]" -background white -alpha remove pertussis-filled.png
 convert -density 300 "1_Pertussis.scanned paper_Redacted.pdf[0]" -background white -alpha remove pertussis-scanned.png