BigInt: cleanup assertion messages (#1549)

test/language/expressions/addition/bigint-and-number.js

test/language/expressions/addition/bigint-arithmetic.js

test/language/expressions/addition/bigint-errors.js

test/language/expressions/addition/bigint-toprimitive.js

test/language/expressions/addition/bigint-wrapped-values.js

test/language/expressions/bitwise-and/bigint-and-number.js

test/language/expressions/bitwise-and/bigint-errors.js

.../language/expressions/bitwise-and/bigint-non-primitive.js

test/language/expressions/bitwise-and/bigint-toprimitive.js

...language/expressions/bitwise-and/bigint-wrapped-values.js

.../language/expressions/bitwise-not/bigint-non-primitive.js

test/language/expressions/bitwise-or/bigint-and-number.js

test/language/expressions/bitwise-or/bigint-errors.js

test/language/expressions/bitwise-or/bigint-non-primitive.js

test/language/expressions/bitwise-or/bigint-toprimitive.js

.../language/expressions/bitwise-or/bigint-wrapped-values.js

test/language/expressions/bitwise-xor/bigint-and-number.js

test/language/expressions/bitwise-xor/bigint-errors.js

.../language/expressions/bitwise-xor/bigint-non-primitive.js

test/language/expressions/bitwise-xor/bigint-toprimitive.js