When an export conflict prevents accessing a special remote, be clearer
about what the problem is and how to resolve it.
This commit was sponsored by Trenton Cronholm on Patreon.
Actual problem is the keyName was set to "Ref \"sha\"", which led to
this follow-on failure since it contained a space.
The bad data would also get into the export database when exporting to a
non-external special remote. Looking briefly at that, I don't think the bad
data will lead to anything more than a re-upload of the file content
now that the problem has been fixed.
This commit was sponsored by Peter Hogg on Patreon.
New table needed to look up what filenames are used in the currently
exported tree, for reasons explained in export.mdwn.
Also, added smart constructors for ExportLocation and ExportDirectory to
make sure they contain filepaths with the right direction slashes.
And some code refactoring.
This commit was sponsored by Francois Marier on Patreon.
