Add better failure handling

With this change the failure handling is improved by printing the jq
error in red color, printing the last successful jq query (meaning a
query that didn't result in a non-zero exit code), and prints the result
to the last successful query as well.

Idea was proposed by @bazzargh on hackernews:
https://news.ycombinator.com/item?id=38191147

bin/jq-repl

@@ -5,13 +5,16 @@
 # if 1st arg is anything else, treat it as a file
 set -eu
 
-JQ_REPL_JQ="${JQ_REPL_JQ:-jq}"
+export JQ_REPL_JQ="${JQ_REPL_JQ:-jq}"
 
 if [ -n "${1:-}" ] && [ "$1" != "-" ] && [ "$1" != "--" ]; then
   input="$1"
+  output=$(mktemp)
+  trap 'rm -f "$output"' EXIT
 else
   input=$(mktemp)
-  trap 'rm -f "$input"' EXIT
+  output=$(mktemp)
+  trap 'rm -f "$input" "$output"' EXIT
 fi
 
 if [ -z "${1:-}" ] || [ "$1" = "-" ]; then
@@ -34,7 +37,7 @@ fi
 
 eval "$FZF_JQ_REPL_COMMAND" |
   fzf \
-    --preview "$JQ_REPL_JQ --color-output ${JQ_REPL_ARGS:-} {q} \"$input\"" \
+    --preview "jq-repl-preview {q} $input $output" \
     --preview-window="down:90%" \
     --height="99%" \
     --query="." \

bin/jq-repl-preview

@@ -0,0 +1,32 @@
+#!/bin/bash
+set -euo pipefail
+JQ_REPL_JQ="${JQ_REPL_JQ:-jq}"
+
+query=$1
+input_file=$2
+last_success_file=$3
+
+out="$last_success_file.new"
+trap 'rm -f "$out"' EXIT
+
+# for colorizing stderr
+red() {
+	while read -r line; do
+		printf "\033[31m%s\033[0m\n" "$line"
+	done
+}
+
+# prints preview query result to stdout, colorizes stderr to red
+do_query() {
+	$JQ_REPL_JQ --color-output ${JQ_REPL_ARGS:-} "$query" "$input_file"
+} 2> >(red)
+
+echo "Last successful jq expression: $query" | red >"$out"
+echo "Last success output:" | red >>"$out"
+if do_query | tee -a "$out"; then
+	# save successful query plus the result
+	mv "$out" "$last_success_file"
+else
+	# print previously saved successful query
+	cat "$last_success_file"
+fi