Added unit tests for error messages
Added unit tests for error messages

This is an attempt to actually exercise all the code paths which can lead to
a ParserError exception (from malformed user input, without tinkering with
any KaTeX internals or exploiting a KaTeX bug).  It documents the current
state of affairs, without changing any error messages.  Comments indicate
future work, particularly with respect to the position often associated with
these error messages.

kwangkim/katex-npm-0.7.0-pre,Khan/KaTeX,kwangkim/katex-npm-0.7.0-pre,kevinbarabash/KaTeX,azotova/KaTeX,xymostech/KaTeX,kwangkim/katex-npm-0.7.0-pre,kwangkim/KaTeX,kwangkim/KaTeX,xymostech/KaTeX,xymostech/KaTeX,Khan/KaTeX,saschatimme/KaTeX,kevinbarabash/KaTeX,Khan/KaTeX,kwangkim/KaTeX,kwangkim/katex-npm-0.7.0-pre,azotova/KaTeX,kwangkim/KaTeX,saschatimme/KaTeX,azotova/KaTeX,azotova/KaTeX,kevinbarabash/KaTeX,xymostech/KaTeX,kevinbarabash/KaTeX,saschatimme/KaTeX,azotova/KaTeX,saschatimme/KaTeX,xymostech/KaTeX,saschatimme/KaTeX,kwangkim/katex-npm-0.7.0-pre,kwangkim/KaTeX,kevinbarabash/KaTeX