Add error checks for common mistakes

[bsuth]

It would be helpful to add error checks for certain common mistakes:

• ~= vs != (Lua vs Erde)
• elseif vs else if

[bsuth]

closed by https://github.com/erde-lang/erde/commit/ab859d9a94b80a81b5516efecfcaf7a5ad9e3cda