Do not give repeated errors on missing type/methods optionally

[Jand42]

Currently, when attempting to compile a .NET library for WebSharper fully, if any basic type is not proxied (does not have a WebSharper/JavaScript implementation), every use of the type or any of its methods generate an error. This can be a long list with many repetitions although possibly there are only a couple missing types.

As a quick solution, these errors could be tracked and only reported at the first occurrence. This is not compromising build result but gives a much shorter list of errors to work through.

[Jand42]

Thinking about this a bit more, when the goal is not to proxy missing types but to work through the code and eliminate the type (switch to using something else), original error reporting (all locations) is better. So I am making it a compiler setting, with old behavior kept by default.