Installer should fail gracefully if module not found.

Weird. I see that there's a drupal_verify_profile function that already should be doing this but isn't getting called for some reason.

Ok. So it is getting called, we're just not doing anything about it:

  // Perform actual installation defined in the profile.

  # NOTE: If one or more modules are missing, $modules will be FALSE. However,
  # we don't check for whether or not drupal_verify_profile returned anything, we just
  # happily pass it on and then finish the installation.
  $modules = drupal_verify_profile($profile, $install_locale);
  drupal_install_profile($profile, $modules);

  // Warn about settings.php permissions risk
  $settings_file = './'. conf_path() .'/settings.php';
  if (!drupal_verify_install_file($settings_file, FILE_EXIST|FILE_READABLE|FILE_NOT_WRITABLE)) {
    drupal_set_message(st('All necessary changes to %file have been made, so you should now remove write permissions to this file. Failure to remove write permissions to this file is a security risk.', array('%file' => $settings_file)), 'error');
  }

  // Show end page.
  install_complete($profile);

I tried throwing the rest of the function in an if ($modules) thing but I'm not sure what to do in the "else" case. Any ideas?

Ok, maybe something like this.

I'm marking this critical because this seems like a major bug in the install system. The installer should not continue if required modules are not found, and the end user should definitely never be exposed to huge scary errors like that.

Sure, makes sense.

This patch places the message in paragraph tags, and also makes "try again" a link back to the same page; it's not intuitive that you would refresh the page to try again.

For some reason I thought you treated "one or more" as singular, but in Googling there seem to be more instances of using it as plural, so changed it. :)

Also, yes, the error messages (set as 'error' drupal_set_message()s) say:

"The blah module is required but was not found. Please move it into the modules subdirectory."

Tentatively setting this back to RTBC again.