Error interface improvements in C

[martukas]

Need to bring up the following with HDF group (observations from #300):

1. If H5E_error2_t could be made to initialize its C-string pointers as null. Would help avoid the problem of exceptions while in the error handling block. We do need to transfer those strings to std::string. I don't think that's unreasonable.
2. Getting messages for major and minor error numbers is itself prone to creating another error stack, in effect overwriting the existing one. This is as problem, because we do want to collect all the error information immediately, not at some later point. The way it is now is like shooting yourself in the foot.
3. No way to determine the state of the error auto-printing mechanism in the C-API without producing more errors, again overwriting the error stack. Every time we generate an exception based on the C-API error stack we need to check this state. If it didn't work like this, we could avoid using the singleton.