GetCompnent's prompts about missing tools should be more informative

[Roland Haas]

Looking at http://lists.einsteintoolkit.org/pipermail/users/2019-June/006940.html the current prompt for a missing subversion client is

You have requested a subversion checkout, but the system was unable to
find subversion.
Please enter the path to subversion:

which is not ideal for multiple reasons:

1. it should name the name of the missing executable “svn” and not the generic software name
2. it should not say “the system” but “GetComponents” or “I”
3. It should contain a note along the lines “if you do not have a subversion client installed, please abort and install the subversion package using your system’s package manager”