fix: Adapt to Stan change: consistent error logging

Commit 5f4f01f74ada379e698e95784495c9d2a6853918 in stan-dev/stan makes
the following change:

> Currently, the services functions are inconsistent on whether or not
> they will throw an exception. The versions which accept num_chains
> should never throw, but the older ones will throw if initialization
> fails. This consistently puts utils::initialize in a try block.

> Additionally, we are currently using logger.info for messages which
> are errors, and in several places not logging an error where we could
> (e.g., the catch blocks accompanying the above trys).

This commit updates httpstan code to adapt to this change.

httpstan/services_stub.py

@@ -157,28 +157,28 @@ async def call(
         fh.close()
     httpstan.cache.dump_fit(b"".join(compressed_parts), fit_name)
 
-    # if an exception has already occurred, grab relevant info messages, add as context
+    # if an exception has already occurred, grab relevant error messages, add as context
     exception = future.exception()
     if exception and len(exception.args) == 1:
         import gzip
         import json
 
         original_exception_message = exception.args[0]  # e.g., from ValueError("Initialization failed.")
-        info_messages_for_context = []
+        error_messages_for_context = []
         num_context_messages = 4
 
         jsonlines = gzip.decompress(b"".join(compressed_parts)).decode()
         for line in jsonlines.split("\n")[:num_context_messages]:
             try:
                 message = json.loads(line)
-                info_message = message["values"].pop().replace("info:", "")
-                info_messages_for_context.append(info_message.strip())
+                error_message = message["values"].pop().replace("error:", "")
+                error_messages_for_context.append(error_message.strip())
             except json.JSONDecodeError:
                 pass
-        # add the info messages to the original exception message. For example,
+        # add the error messages to the original exception message. For example,
         # ValueError("Initialization failed.") -> ValueError("Initialization failed. Rejecting initial value: Log probability ...")
-        if info_messages_for_context:
-            new_exception_message = f"{original_exception_message} {' '.join(info_messages_for_context)} ..."
+        if error_messages_for_context:
+            new_exception_message = f"{original_exception_message} {' '.join(error_messages_for_context)} ..."
             exception.args = (new_exception_message,)
 
     # `result()` method will raise exceptions, if any