deal with attempt to export filename with # or ? to webdav

xporting files with '#' or '?' in their name won't work because urls get
truncated on those. Fail in a better way in this case, and avoid failing
when removing such files from the export, so after the user has renamed the
problem files the export will succeed.

doc/bugs/webdav_remote_fails_on_pound_signs/comment_1_b6b1cff5972a655d14b592652ae8d1a8._comment

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+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 1"""
+ date="2019-02-07T17:11:22Z"
+ content="""
+I guess that "#" and "?" in a webdav path is just not possible, since
+the path becomes part of the url that is used. It might even be legal for a
+webdav server to cut those off the url and store data to the basename,
+which could overwrite another file that shared that basename.
+
+I don't like escaping those somehow, because the user should expect to see
+the same filenames in the export that they have in their tree. So exporting
+should fail. And removal should can succeed without doing anything, to
+let it recover, which is ok since the file content is certianly not located
+at an url containing either of those characters.
+
+I think that S3 might have the same problem, have not tried it yet.
+"""]]

doc/bugs/webdav_remote_fails_on_pound_signs/comment_2_a88aa0458413481787b4a59990a210df._comment

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+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 2"""
+ date="2019-02-07T17:47:06Z"
+ content="""
+Fixed for webdav, still need to check S3.
+"""]]