Improve error message for tea . in non-venvs

Refs pkgxdev#133

src/app.exec.ts

@@ -33,6 +33,13 @@ export default async function exec(opts: Args) {
 
   try {
     if (cmd.length) {
+      if (cmd[0] == '.') {
+        // if we got here and `.` wasn’t converted into something else
+        // then there was no default target or no exe/md and running `.`
+        // will just give a cryptic message, so let’s provide a better one
+        throw new TeaError('not-found: exe/md: default target', {opts})
+      }
+
       await run({ cmd, env })  //TODO implement `execvp` for deno
     } else if (opts.pkgs.length) {
       await repl(installations, env)

src/utils/error.ts

@@ -52,12 +52,12 @@ export default class TeaError extends Error {
       msg = `target \`${ctx.name}' contains no script region`
       break
     case 'not-found: exe/md: default target':
-      msg = undent`
-        default target not found:
-
-            ${ctx.requirementsFile}
-        `
-        break
+      if (ctx.requirementsFile) {
+        msg = `default target not found in \`${ctx.requirementsFile}'`
+      } else {
+        msg = "no `README' or `package.json' found"
+      }
+      break
     case 'http':
       msg = ctx.underr?.message ?? "contact violated"
       break