.arcconfig

config: move message about leading spaces in config to config.py

When the config parser raises a ParseError, it uses the line that
failed to parse as the error message. It doesn't currently tell the
user anything about why it failed to parse. b13b99d39a46 (config:
highlight parse error caused by leading spaces (issue3214),
2014-03-16) added a checked based on the error *message* having
leading spaces. That has worked fine because only the config parser
uses the line itself as error message (I think the revset and fileset
parsers use more user-friendly proper messages). It still feels like a
hack. Let's make the config parser give a useful message about leading
whitespace instead. We should ideally follow up with more useful
messages for other parse errors in config files.

Differential Revision: https://phab.mercurial-scm.org/D9241

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