py: Allow lexer to raise exceptions during construction.
This patch refactors the error handling in the lexer, to simplify it (ie reduce code size). A long time ago, when the lexer/parser/compiler were first written, the lexer and parser were designed so they didn't use exceptions (ie nlr) to report errors but rather returned an error code. Over time that has gradually changed, the parser in particular has more and more ways of raising exceptions. Also, the lexer never really handled all errors without raising, eg there were some memory errors which could raise an exception (and in these rare cases one would get a fatal nlr-not-handled fault). This patch accepts the fact that the lexer can raise exceptions in some cases and allows it to raise exceptions to handle all its errors, which are for the most part just out-of-memory errors during construction of the lexer. This makes the lexer a bit simpler, and also the persistent code stuff is simplified. What this means for users of the lexer is that calls to it must be wrapped in a nlr handler. But all uses of the lexer already have such an nlr handler for the parser (and compiler) so that doesn't put any extra burden on the callers.
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