`\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 = [1] if c.length > 1`.
Minor tweak: `qwe if rty` form instead of multiline when `qwe` is only one line long.
Minor tweak: thanks to `#{}` interpolation we can omit `.to_s`.