[BUG] 0.14 -- Improve error checking in of case "-r1:10"

[Jari Aalto]

It happens, that sometimes a svn style versio numbering is written:

    $ bzr diff -r7:10 Html.pm
    bzr: ERROR: No WorkingTree exists for file:///test/.bzr/checkout/.

This is a bit hairy to understand. Could this be made to read:

    $ bzr diff -r7:10 Html.pm
    bzr: ERROR: No WorkingTree exists for file:///test/.bzr/checkout/.
    bzr: H, -r7:10 looks like SVN style range. Did you mean -r7..10?

Jari