nicer reporting of unparseable version numbers in deps

[zooko]

As described in #1387, if one of the dependencies has a version number that is not considered "Rational" by verlib, Tahoe-LAFS emits

  File "/Users/zooko/playground/tahoe-lafs/pristine/src/allmydata/__init__.py", line 375, in <module>
    check_all_requirements()
  File "/Users/zooko/playground/tahoe-lafs/pristine/src/allmydata/__init__.py", line 373, in check_all_requirements
    raise PackagingError(get_error_string(errors, debug=True))
allmydata.PackagingError: 
TypeError: expected string or buffer

followed by a potentially large and messy-looking dump of the PYTHONPATH.

It would be better if the error message said that the problem was that a version number couldn't be parsed, as well as specifying what the version number was that couldn't be parsed and where it came from.

[davidsarah]

It's a bug in verlib that it raises TypeError rather than IrrationalVersionError.

I also agree that it is a bug in check_all_requirements (or check_requirement) that the error doesn't say that a version number couldn't be parsed, what the number was, or where it came from.

However, it's not clear to me that it's wrong to print the large and messy-looking PYTHONPATH dump. The purpose of that dump is to be able to debug path problems on users' machines that might otherwise be difficult to reproduce.

[davidsarah]

allmydata/init.py: Nicer reporting of unparseable version numbers in dependencies. fixes #1388

[zooko]

attachment:nicely-report-unparseable-versions.darcs.patch​ Looks good! Thanks! :-)

(I guess this patch doesn't need a NEWS entry.)

[david-sarah@…]

In f3246a9ca013807a:

allmydata/init.py: Nicer reporting of unparseable version numbers in dependencies. fixes #1388