postem: No error messages displayed in Post 'Em.

Issue: Post 'Em is supposed to give alerts when one of the following actions is taken:

1. Trying to upload a file that is not .csv file type. You should get an alert, "Alert: The file [filename] that you uploaded is not a valid file type. Please make sure your file is saved as a *.csv file and try again."

2. Trying to upload a file with a username of a student not in the site. You should get an alert, "Alert: The following usernames are not associated with participants in this site: [usernames listed] To continue, you must first add these participants to the site or remove the participants from the file."

3. Clicking Post before selecting a file to upload. You should get an alert, "Alert: Please choose a file to post."
   In Sakai 11, none of these produces an error message for the user. The screen just refreshes. If a file with an incorrect mimetype or a csv file with an incorrect user listed was selected, its filename continues to display on the screen after it refreshes.

Steps to reproduce issue 1 (selecting file that is not .csv):

1. Go to PostEm and click Add.

2. Enter a Title

3. Click Choose a CSV file or URL from Resources.

4. Click Browse and select a file from your computer that is a different format than CSV (e.g. an image or .doc file).

5. Click Continue.

6. The page reloads, there is no error message indicating that you are trying to upload an incorrect file type:

7. This is a regression from 10. In 10, you get an error message indicating, "Alert: The file [filename] that you uploaded is not a valid file type. Please make sure your file is saved as a *.csv file and try again."

Steps to reproduce issue 2 (selecting file that has incorrect students listed):

1. Create a CSV file with some students listed who are not members of the site.

2. Go to PostEm and click Add.

3. Enter a Title.

4. Click Choose a CSV file or URL from Resources.

5. Click Browse and select the file you created in step 1.

6. Click Continue.

7. The page reloads, there is no error message indicating that you are trying to upload a file with students not in the site:

8. This is a regression from 10. In 10, you get an error message indicating, "Alert: The following usernames are not associated with participants in this site: [usernames listed] To continue, you must first add these participants to the site or remove the participants from the file. "

Steps to reproduce issue 3 (clicking Post with no file selected): (This issue is on https://sakaiproject.atlassian.net/browse/SAK-31784#icft=SAK-31784)

1. Go to PostEm and click Add.

2. Enter a Title.

3. Click Post.

4. You should get an error message: "Alert: Please choose a file to post." but you don't.