It is called just that: a "mild oath". Such things as "Egad!" or "Gosh darnit!" or "Great Jehosophat!".
A mild oath that starts with "e" is usually "egad." Perhaps the "m" is incorrect.
vow
The mild oath to express anger used by Mercutio is "zounds," which is a minced oath for "God's wounds." It was a common expletive in Shakespearean times, used as an expression of frustration or anger.
drat, egad
drat
the answer is 'zounds'
heck
Dang?
Berroga means Used as a mild oath.
drat
Heck.