Gracefully handle LexicalError's from semgrep-core

[mschwager]

E.g.

$ python3 -m semgrep --pattern '$X == $X' --lang c /tmp/test.c --verbose
Warning: Semgrep encountered a lexical error when running - on /tmp/test.c. Please ensure this is valid code.
ran 1 rules on 1 files: 0 findings
1 files could not be analyzed; run with --verbose for details or run with --strict to exit non-zero if any file cannot be analyzed

$ python3 -m semgrep --pattern '$X == $X' --lang c /tmp/test.c --json
{"results": [], "errors": [{"type": "LexicalError", "code": 11, "path": "/tmp/test.c", "rule_id": "-"}]}
ran 1 rules on 1 files: 0 findings
1 files could not be analyzed; run with --verbose for details or run with --strict to exit non-zero if any file cannot be analyzed

[github-actions]

You added quite a few lines in semgrep, but didn't update the changelog. Consider doing so!

[nbrahms]

NICE!!!!!!!!