Evaluation returns for bad sequence rather than throwing an error

**Tool:**
Sequence Builder, particularly build via text, but maybe others.

**Bug Description:**
Sequences with a bad terminal pass evaluation rather than returning an error. The terminal that is known to cause this behavior (there might be others) has termination like: DManpa1-4-OH

See linked bug report for a longer sequence that returns a successful evaluation complete with rotamer options.

The build fails, but with no information why, not even in the build directories server-side. I had to scrape GEMS error logs to figure it out.

**To Reproduce:**
Please give us as many details as possible, so we can reproduce the bug. Bugs that cannot be reproduced are challenging to fix.

1. Go to glycam.org/txt
2. Enter a sequence that has a bad terminal like the one in the description. Here's one to try: DGalpb1-6DManpa1-4-OH
3. Hit submit.
4. If your sequence has rotamers, choose rotamers or accept defaults and hit Generate Selected Structures. Otherwise proceed to 5.
5. Wait endlessly for the job to complete.

**Expected behavior**
The poorly formed sequence should not pass evaluation. Instead, it should return an error.

**Original Bug Report:**
Here is the issue that first reported this. The notes about the system being overwhelmed are true, and I had to get that under control before I could learn the real problem.

https://github.com/GLYCAM-Web/website/issues/285