replace user errors with exceptions

Exceptions are better to handle than errors. What I don't like is that
we now have an unfortunate mix of return code and exception signalling
for errors. Some met

replace user errors with exceptions

Exceptions are better to handle than errors. What I don't like is that
we now have an unfortunate mix of return code and exception signalling
for errors. Some methods still return false for errors while others
now throw exceptions (always returning true otherwise).

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