Print a better error when workspace path contains the File.pathSeparator

[Gedochao]

Fixes #1411

: is the File.pathSeparator on UNIX systems, used as the separator on classpaths by the compiler.
Unfortunately, it seems that there is no way of escaping it, even though : is a valid character for directory names.
The recommended workaround is to either move your project to a sane path or at the very least override your workspace with one.

scala-cli run Hello.scala
# [error]  Invalid workspace path: ~/IdeaProjects/scala-cli-tests/weird:directory
# Workspace path cannot contain a : (colon character).
# Consider moving your project to a different path.
# Alternatively, you can force your workspace with the '--workspace' option:
#     scala-cli run --workspace <alternative-workspace-path> Hello.scala

[lwronski]

LGTM.