dygraph-utils.js: isElementContainedBy -> isNodeContainedBy

This fixes a type nit when using the Closure Compiler, since walking
parentNode can give an Element or Document node, not just an Element.
dygraph-tickers.js: Fix variable redeclaration
dygraph-utils.js: Remove trailing white space
dygraph-options.js: Remove unused labels_ field

This fixes a warning caught by Closure Compiler. This private field was
never written to, and it was read by only a single, unused method.
dygraph-utils.js: Make Dygraph.error and Dygraph.update public

These seem like reasonable utility functions, and they were used outside
of Dygraph (specifically in DygraphOptions). Making these non-private
resolves Closure Compiler warnings inside dygraph-options.js.
Make dygraph-options.js warning-free under Closure Compiler