Status code for multiple responses in subject endpoint

**Describe the bug**
The subject endpoint currently returns a response with 500 status code when a subject_id input results in Multiple found. It would be better for it to return a 406 or a status in the 400s since this is not actually a server error

**To Reproduce**
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2. Click on '....'
3. Scroll down to '....'
4. See error

**Expected behavior**
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**Screenshots**
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**Desktop (please complete the following information):**
- OS: [e.g. iOS]
- Browser [e.g. chrome, safari]
- Version [e.g. 22]

**Smartphone (please complete the following information):**
- Device: [e.g. iPhone6]
- OS: [e.g. iOS8.1]
- Browser [e.g. stock browser, safari]
- Version [e.g. 22]

**Additional context**
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