Help icon at the left side of "pgr_f()" label doesn't work due to Python error

[sanak]

Problem
Help icon at the left side of function dropdown doesn't work due to Python error

To Reproduce

1. Click help (?) icon at the left side of pgr_f() label
2. Python error occurs

Expectation
The errors should not happen and the help should be shown.

Platform/versions

• OS: OSGeoLive 15 rc1
• pgRoutingLayer: 3.0.1

[sanak]

Well, this error happen when DB connection doesn't exist (#140), but I think that it's better to catch KeyError exception safely, so I created the PR #145 for that.