This commit introduces ErrorReporter, a utility class which consolidates
all of the error logging functionality into a common set of functions.
It also replaces all direct interactions with an ErrorList with calls to
an ErrorReporter.
This commit resolves issue #2209
The unary '+' serves no meaningful purpose in Solidity and it makes it
possible to produce typos with dagerous implications (e.g. 'a =+5 '),
so we are deprecating it. The SyntaxChecker currently issues warnings
on the unary '+' but will still compile it for now.
