Report more meaningful errors on why virtualenv creation failed

[vlaci]

virtualenv fails if the destination directory contains unicode characters or spaces. A more descriptive error is printed if it's likely that that was the case.
As long as the upstream problem exists #121 cannot be fixed.

Open questions:

• Should hard-fail even before executing virtualenv if the destination directory contains unsupported characters? -- I choosed not to in order to not block support of such paths if upstream is fixed
• Should check for unsupported characters and only print error message about them if they are found? -- I decided not to, because testing this part of the code would be very hard and I don't like the idea to introduce logic without tests. I could abstract away the error reporting of venv.update() calls and test it in isolation if needed.

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[vlaci]

#203 is also related

[obestwalter]

Thanks @vlaci :)