importlib.abc.SourceLoader.get_source() was re-raising SyntaxError and

importlib.abc.SourceLoader.get_source() was re-raising SyntaxError and
UnicodeDecodeError as ImportError. That was over-reaching the point of
raising ImportError in get_source() (which is to signal the source
code was not found when it should have). Conflating the two exceptions
with ImportError could lead to masking errors with the source which
should be known outside of whether there was an error simply getting
the source to begin with.