reconsider error handling

[jll63]

Make throwing exceptions the default?

[jll63]

I think the current way is the right way. Exceptions should be used to report errors that occur during the execution of a correct program. The errors that the library reports are actually bugs in the program. They are in the same category as inadvertently calling a pure virtual function in a constructor. This typically results in a brief error message, then program termination via abort. Also throwing exceptions (by default) to report errors would not work if exceptions are disabled. Thus the default would have to be different.