* FTSEurofirst 300 down 0.1 pct, Euro STOXX 50 down 0.2 pct* Barclays, Aberdeen weigh after weak updates* DSM, Solvay boosted by brighter outlookBy Francesco CanepaLONDON, May 6 (Reuters) - European stocks edged lower on Tuesday, as outlook statements from chemical groups SolvaySOLB.BR and DSM DSMN.AS were outweighed by a selloff in financial shares after weak results from Barclays BARC.L and Aber