Start: fix bug when custom folder doesn't exist

[heewa]

A small code bug in the Start workbench causes it to fail to load in the case that there is a "custom folder" configured which doesn't exist. This change simply checks before using it, reports a warning if it doesn't, and continues loading the workbench either way.

---

• Your pull request is confined strictly to a single module. That is, all the files changed by your pull request are either in App, Base, Gui or one of the Mod subfolders. If you need to make changes in several locations, make several pull requests and wait for the first one to be merged before submitting the next ones
• In case your pull request does more than just fixing small bugs, make sure you discussed your ideas with other developers on the FreeCAD forum (it's a small bug fix, but I made a forum post anyway: https://forum.freecadweb.org/viewtopic.php?p=502998)
• Your branch is rebased on latest master git pull --rebase upstream master
• All FreeCAD unit tests are confirmed to pass by running ./bin/FreeCAD --run-test 0 (will use CI for testing. is that alright?)
• All commit messages are well-written ex: Fixes typo in Draft Move command text
• Your pull request is well written and has a good description, and its title starts with the module name, ex: Draft: Fixed typos
• Commit messages include issue #<id> or fixes #<id> where <id> is the FreeCAD bug tracker issue number in case a particular commit solves or is related to an existing issue on the tracker. Ex: Draft: fix typos - fixes #0004805

[0penBrain]

Looks good to me. Eventually slightly better to use 'continue' instead of 'else'.

[yorikvanhavre]

good catch.. Thanks!