Fix OMERO-homebrew* jobs when Homebrew installation failed

Simply test existence the $BREW_DIR folder instead of testing the existence of
a full Homebrew installation before wiping out $BREW_DIR.
Explicitely specify Markdown extension for VM scripts Readme file
Add source hyperlinks in the VM readme
Fix codeblocks
Fix Readme to comply with documentation standards
Format variable names
Fix formatting issues

- Add break lines to long code blocks
- Create relative link to VM folder instead of using hardcoded URL
- Remove hardcoded version number for DL_LOC
- Unify utility scripts code block examples with the rest of the README
Throw an exception if delete returns false
Add file delete exception.
Check args for rm and throw appropriate exceptions.
merge develop -D org -I include -E conflict -E needs-fixes -E broken: PR 587 (Modulo retrieve (rebased onto develop))
merge develop -D org -I include -E conflict -E needs-fixes -E broken: PR 634 (Big images fs 9701 (develop))
merge develop -D org -I include -E conflict -E needs-fixes -E broken: PR 645 (xsd-fu cleanup)
merge develop -D org -I include -E conflict -E needs-fixes -E broken: PR 654 (Rebased onto develop of spec clean PR)
merge develop -D org -I include -E conflict -E needs-fixes -E broken: PR 655 (Merge xsd-fu changes from schema-2012-06)
merge develop -D org -I include -E conflict -E needs-fixes -E broken: PR 669 (Introduce FsFile-based representation of checked paths and adjust destination of imported files.)
merge develop -D org -I include -E conflict -E needs-fixes -E broken: PR 672 (Web fs image links)
merge develop -D org -I include -E conflict -E needs-fixes -E broken: PR 687 (License updates)
merge develop -D org -I include -E conflict -E needs-fixes -E broken: PR 690 (Replace accented character in unknown charset with unicode equivalent (#10251))
merge develop -D org -I include -E conflict -E needs-fixes -E broken: PR 693 (Fs import notifications)