assert: wrap original err on `ifError` fail

For assert.ifError fails, wrap the original err in a new AssertionError

Inspiration: https://github.com/nodejs/node/issues/12803#issuecomment-299041508 /cc @nodejs/testing

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

test, assert