Ignore Errors when reading Files

[bakerboy448]

Workaround for german umelts and similar reading utf-8 issues

Closes #413

[adrienverge]

Hello,

Ignoring errors instead of fixing the root problem is generally not the solution adopted by yamllint. Please check other pull requests about encoding problems and especially the ones since yamllint uses io.open(): #238, #240, #218, #285, #347, etc. I believe the problem should be fixed around there!