add more specific error messages for a variety of cases involving inappropriate keyword or register tokens, suggesting an elided token, per #128.
add more specific error messages for a variety of cases involving inappropriate keyword or register tokens, suggesting an elided token, per #128.

james0x0A/Octo,JohnEarnest/Octo,JohnEarnest/Octo,JohnEarnest/Octo,JohnEarnest/Octo,james0x0A/Octo,JohnEarnest/Octo