fix(emacs/lean-type): improve displaying proof state messages

 - Do not repeat the same message
 - Do not display the empty message
 - Do not display "[stale]" for proof states
fix(library/definitional/projection): remove redundant 'error in'
fix(frontends/lean/info_manager): suppress useless tactic type information, closes #277
fix(emacs/lean-flycheck): should accept error messages with empty lines
feat(library/tactic/proof_state): add empty line between goals, closes #281
fix(frontends/lean/elaborator): incorrect error position in begin-end block, fixes #276