Fix/no genders error

[DavidMarchant]

Fixes #71 and #81 when merged

• Move all __nodeattr code into a single method with more params so it can be surrounded by a single try-except block
• intercept any invalid nodenames before this execution is tried to a) prevent injection attacks & b) give more informative error msgs

How do you feel about the use of regex to sanitize nodenames? William mentioned using click but I wasn't really sure what how I would go about doing that Did these in the same branch as they both dealt with the genders file - i know this isn't optimum, sorry

[DavidMarchant]

Ah right! sorry @WilliamMcCumstie, combination of two attempts there, fixed: https://github.com/alces-software/adminware/pull/83/commits/d861777ca0e73d2f113402ede2f675c042ef3ecd