Don't crash fast-import if the marks cannot be exported.
Don't crash fast-import if the marks cannot be exported.

Apparently fast-import used to die a horrible death if we
were unable to open the marks file for output.  This is
slightly less than ideal, especially now that we dump
the marks as part of the `checkpoint` command.

Signed-off-by: Shawn O. Pearce <f01032651c12a22b2103346ba96b3b6b231f9d3c@spearce.org>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git