`'\t'` error msgs bug and some codestyle tweaks

Minor bugs: `\t` were used inside single quoted strings.
Tweak: `a, b = c` is the same as `a = c[0]; b = c[1] if c.length > 1`.
Minor tweak: `qwe if rty` form when `qwe` is only one line long.
Minor tweak: thanks to `#{}` interpolation we can omit `.to_s`.
Minor tweak: `str.index('c')` is logically the same here as `str['c']`.