Make errors more informative

Make error messages from the lexing and parsing stages be a bit more helpful. If
provided with the input and a position, the error will display the error
position, and the nearby input with the error position underlined (yay combining
marks). Also, standardize the errors a bit (remove doubled "Error:" strings)

Test plan:
 - Make sure the errors look totally sweet (before: {F15602}, after: {F15603})
 - Trigger every error (that can be triggered) in Parser, Lexer, and buildTree
   using the inputs:
  `a^`
  `a_`
  `a^x^x`
  `a_x_x`
  `\color f`
  `\blue `
  `\Huge`
  `\llap`
  `\text`
  `\dfrac`
  `\dfrac{x}`
  `\d`
  `\blue{`
  `\color{#f`
  `{\Huge{x}}`
 - See that the tests still work

Auditors: alpert