Add informative message for missing executable on Windows

[shadowspawn]

Problem

There is an informative runtime message to help the author when fail to find a stand-alone subcommand, but the message does not appear on Windows.

See: #2290

Solution

Windows always tries opening the executable as an argument to node. Test for the existence of the file first.

ChangeLog

• add: show informative error on Windows for missing stand-alone executable subcommand

[shadowspawn]

Ouch. The new failure mode was hard to get working with the existing tests, but refactoring into a separate routine allowed suppressing the behaviour so existing tests could stay much the same.

[abetomo]

The method name is _throwForMissingExecutable, but it may not throw, so why not name it something like _validateExecutable?

Or how about doing the checks outside of the method?

if (!fs.existsSync(executableFile)) {
 _throwForMissingExecutable(...);
}

[shadowspawn]

method name

Oh, yes. I originally had the check outside the routine. Will change name.

Or how about doing the checks outside of the method?

I agree that would be tidier for the code and usage. However, I started that way, and multiple unit tests check whether fs.existsSync has been called and broke. I didn't think of an easy way of fixing the tests, other than moving the fs. fs.existsSync inside the routine. Then easy to completely bypass the new code and run the old tests.

[shadowspawn]

We have several _checkForX routines already so following that style, _checkForMissingExecutable.

[shadowspawn]

Lots!

• _checkForMissingMandatoryOptions
• _checkForConflictingLocalOptions
• _checkForConflictingOptions
• _checkForBrokenPassThrough