The query parser (QueryParser.cpp) detects and handles ill-formed queries via thrown ParseError or other RecoverableError exceptions. However, such exceptions are currently not caught by QueryRunner.cpp as part of a spawned thread, causing REPL or any other caller to crash with little to no helpful diagnostic message.

It would be preferable to catch these exceptions and communicate the appropriate error condition as an alternative result within statistics. REPL query could then detect this error result and communicate/handle it appropriately.

Also: I recommend treating a single term phrase as a unigram in QueryParser::ParsePhrase. Thus, instead of treating "term" as an error, this query would be treated as equivalent to term.