Wrap returned errors

[rentziass]

Fixes #56

Wrap errors in:

• internal/dispatcher/dispatcher.go
• internal/dispatcher/task.go
• models/db.go
• pkg/vegeta/options.go
• pkg/vegeta/report.go
• pkg/vegeta/vegeta.go
• internal/reporter/reporter.go

[nitishm]

Looks like you got everything. Just need the minor fixes and then it's good to merge. Great job @rentziass