Exxon Mobil Corp.’s biggest profit miss in at least a decade is the starkest sign yet that major oil explorers remain mired in the deepest market slump in a generation. After resisting the industry trend of discounting the value of oil and natural gas fields that turned into money-losers amid the 2 1/2-year market slump, Exxon capitulated on Tuesday and took a $2 billion hit on the value of some Rocky Mountain gas.