fs: improve error message for invalid flag

[jasnell]

Pull Request check-list

• Does make -j8 test (UNIX) or vcbuild test nosign (Windows) pass with
  this change (including linting)?
• Is the commit message formatted according to [CONTRIBUTING.md][0]?
• If this change fixes a bug (or a performance problem), is a regression
  test (or a benchmark) included?
• Is a documentation update included (if this change modifies
  existing APIs, or introduces new ones)?

Affected core subsystem(s)

fs

Description of change

Flags on fs.open and others can be passed as strings or int.
Previously, if passing anything other than string or int,
the error message would only say that flags must be an int.

[jasnell]

This one isn't obvious... basically, by changing the early out condition on stringToFlag it falls through to the error that is reported if the string flag is not correct.

[cjihrig]

Could you add a test?

[jasnell]

@cjihrig ... done!

[jasnell]

CI: https://ci.nodejs.org/job/node-test-pull-request/1873/
CI: https://ci.nodejs.org/job/node-test-pull-request/1874/

[cjihrig]

LGTM if the CI is happy

[jasnell]

Generally CI is good but there was a build bot error on OSX... trying again: https://ci.nodejs.org/job/node-test-pull-request/1877/

[jasnell]

Ok, looks like something is borked with the OSX build bot (cc @nodejs/build). Otherwise this looks ok. Tests pass locally on OSX. Given that this is a major, I'll give it until tomorrow or Wednesday to land.

[jasnell]

Fixes: #2871

[MylesBorins]

LGTM

[bnoordhuis]

You can shorten this to just the regex, i.e.:

assert.throws(
  () => fs._stringToFlags({}),
  /^Unknown file open flag: \[object Object\]/);

[bnoordhuis]

LGTM with a suggestion.

[jasnell]

Landed in 8bb60e3 and a5aa7c1