fix: captureError would fail on an Error attribute that is an invalid date

[trentm]

Fixes: #2030

---

Example of the tests failing without the fix:

...
# #attributesFromErr()
# no attrs
ok 39 got expected attrs
# string attr
ok 40 got expected attrs
# Invalid Date attr
/Users/trentm/el/apm-agent-nodejs6/lib/errors.js:99
          val = val.toISOString()
                    ^

RangeError: Invalid time value
    at Date.toISOString (<anonymous>)
    at attributesFromErr (/Users/trentm/el/apm-agent-nodejs6/lib/errors.js:99:21)
    at Test.<anonymous> (/Users/trentm/el/apm-agent-nodejs6/test/errors.test.js:366:21)
    at Test.bound [as _cb] (/Users/trentm/el/apm-agent-nodejs6/node_modules/tape/lib/test.js:99:32)
    at Test.run (/Users/trentm/el/apm-agent-nodejs6/node_modules/tape/lib/test.js:117:31)
    at Test.bound [as run] (/Users/trentm/el/apm-agent-nodejs6/node_modules/tape/lib/test.js:99:32)
    at Test._end (/Users/trentm/el/apm-agent-nodejs6/node_modules/tape/lib/test.js:220:11)
    at Test.bound [as _end] (/Users/trentm/el/apm-agent-nodejs6/node_modules/tape/lib/test.js:99:32)
    at Test.<anonymous> (/Users/trentm/el/apm-agent-nodejs6/node_modules/tape/lib/test.js:219:40)
    at Test.emit (node:events:527:28)

and they pass after.

[ghost]

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• Start Time: 2022-08-10T22:10:26.374+0000

• Duration: 21 min 51 sec

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Test	Results
Failed	0
Passed	258342
Skipped	0
Total	258342

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