Display error if database name can't be determined.
Display error if database name can't be determined.

According to issue #650, database name **is** required for some commands (in particular migrate). There is currently no check that this required parameter is actually set. This change would address that issue.
rquadling/phinx,daniel-gomes-sociomantic/phinx,dankempster/phinx,courtney-miles/phinx,robmorgan/phinx,aCodify/phinx,DeltaPHP/phinx,Thamrongsak/phinx,ikagawa/phinx,aCodify/phinx