improves error handling

[dmalan]

• Color-codes log
  • Green if successful
  • Yellow if valid syntax but constraint violation
  • Red if invalid syntax
• Requires that *.db exist for SQLite, so that students don't accidentally create an empty DB
• Catches SQLAlchemy exceptions, outputs only their error messages.
• Implements check whether sqlite:/// file exists, throw exception if not #40.
• Capitalizes comments for style

[dmalan]

@crossroads1112 any concerns?