"Run Assembler" does not indicate progress or errors

[chrstphrchvz]

When "Run Assembler" is clicked, nothing happens until the server responds with the assembled code and the editor page updates the text areas (list file and assembler messages).

And if the editor page is unable to connect to the server, the page quietly fails, and an error such as the following is printed to the console:

TypeError: NetworkError when attempting to fetch resource.  (unknown)

Because this whole process takes a few seconds, there should instead be an indicator to the user of some activity. For example, display an "Assembling…" message and changing the mouse cursor to progress or wait. The user should also be notified in case the file could not be assembled, whether due to connectivity issues or server problems. See also #10: for troubleshooting purposes, any CGI script errors should be displayed somewhere on the editor page.

[chrstphrchvz]

Closed in favor of #16: no longer using CGI, but the general issue remains.