fix: catch exceptions due to failure to create XML from text

I don't know what causes this, but we got a few 5xx errors from the
Heroku logs with this trace:
```
   File "/app/readalongs/web_api.py", line 209, in assemble
     parsed = parse_xml(
              ^^^^^^^^^^
   File "/app/readalongs/text/util.py", line 138, in parse_xml
     return etree.fromstring(
            ^^^^^^^^^^^^^^^^^
 lxml.etree.XMLSyntaxError: PCDATA invalid Char value 3, line 8, column 26
```

While it makes no sense we should fail to parse XML we just generated
in this broader block:
```
    parsed = parse_xml(
        create_ras_from_text(
            (request.input_text or "").splitlines(keepends=True),
            text_languages=request.text_languages,
        )
    )
```
let's just put a try/except in there since we saw it live on Heroku.

Whatever the error in the input is, though I cannot reproduce it, should
generate a 422 HTTP code.

readalongs/web_api.py

@@ -206,12 +206,18 @@ async def assemble(
                     ),
                 )
         elif request.mime_type == InputFormat.TEXT:
-            parsed = parse_xml(
-                create_ras_from_text(
-                    (request.input_text or "").splitlines(keepends=True),
-                    text_languages=request.text_languages,
+            try:
+                parsed = parse_xml(
+                    create_ras_from_text(
+                        (request.input_text or "").splitlines(keepends=True),
+                        text_languages=request.text_languages,
+                    )
                 )
-            )
+            except etree.ParseError as e:
+                raise HTTPException(
+                    status_code=422,
+                    detail="XML could not be created from the provided text",
+                ) from e
 
         else:  # pragma: no cover
             raise HTTPException(