Improve error handling of html-tty(1) executable

It's too easy to forget this program is a post-processor, and pass files
which haven't yet been processed by troff(1). Diagnostics will receive a
more detailed explanation in the program's (yet-to-be-written) man page.

bin/html-tty

@@ -14,25 +14,35 @@ new Promise(resolve => {
  });
  }
 }).then(data => {
+
+ // If input isn't ditroff, bail with a meaningful error message
+ if(!/^x\s*T\s+\S/.test(data.replace(/^\s*#.*$/gm, "").trim())){
+ const reason = /(?:^|\n)\.\S+/.test(data)
+ ? "input appears to be unprocessed Roff source"
+ : "input does not contain a ditroff(7) header";
+ process.stderr.write(`html-tty: Aborting, ${reason}\n`);
+ process.exit(2);
+ }
+
  const htmlTTY = new (require("..").TTYRenderer)();
  const isRaw = /\x08/.test(data);
  let output = htmlTTY.process(data, isRaw);
  if(process.stdout.isTTY)
  output = output
- .replace(/^\n+/, "")
- .replace(/<b>/g, "\x1B[1m")
- .replace(/<u>/g, "\x1B[4m")
- .replace(/<\/b>/g, "\x1B[22m")
- .replace(/<\/u>/g, "\x1B[24m")
- .replace(/<b data-sgr="(\d+)"[^>]+>/g, "\x1B[1;38;5;$1m")
- .replace(/<u data-sgr="(\d+)"[^>]+>/g, "\x1B[4;38;5;$1m")
- .replace(/<span data-sgr="(\d+)"[^>]+>/g, "\x1B[38;5;$1m")
- .replace(/<\/span>/g, "\x1B[39m")
- .replace(/<a[^>]+>|<\/a>/g, "")
- .replace(/&lt;/g, "<")
- .replace(/&gt;/g, ">")
- .replace(/&amp;/g, "&")
- .replace(/&#(\d+);/g, (_,c) => String.fromCharCode(c));
+  .replace(/^\n+/, "")
+  .replace(/<b>/g, "\x1B[1m")
+  .replace(/<u>/g, "\x1B[4m")
+  .replace(/<\/b>/g, "\x1B[22m")
+  .replace(/<\/u>/g, "\x1B[24m")
+  .replace(/<b data-sgr="(\d+)"[^>]+>/g, "\x1B[1;38;5;$1m")
+  .replace(/<u data-sgr="(\d+)"[^>]+>/g, "\x1B[4;38;5;$1m")
+  .replace(/<span data-sgr="(\d+)"[^>]+>/g, "\x1B[38;5;$1m")
+  .replace(/<\/span>/g, "\x1B[39m")
+  .replace(/<a[^>]+>|<\/a>/g, "")
+  .replace(/&lt;/g, "<")
+  .replace(/&gt;/g, ">")
+  .replace(/&amp;/g, "&")
+  .replace(/&#(\d+);/g, (_, char) => String.fromCharCode(char));
  process.stdout.write(output + "\n");
 }).catch(error => {
  console.error(error);