Throw descriptive error when 'subscribable' of undefined

[thgh]

Fix #2139

Should there be a test for this?

[codecov-io]

Codecov Report

Merging #2155 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2155   +/-   ##
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  Coverage   91.83%   91.83%           
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  Files           1        1           
  Lines          49       49           
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  Hits           45       45           
  Misses          4        4

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[thgh]

Will update PR with this.error

[thgh]

Updated 😃

[thgh]

The tests are green now :-)

[Conduitry]

In case we want to add more $$-prefixed variables, what we're doing elsewhere is checking that name[0] === '$' && name[1] !== '$'.

I think we probably also want to indicate in the error code/message that this is a temporary state of affairs, and that subscribing to global stores will be supported someday when we get around to it. I'm using the not-implemented code in another PR but it's not merged yet and I'm not sure that's a convention that will get off the ground.

[Rich-Harris]

Thanks @thgh — I think @Conduitry is right that it should be possible to subscribe to global stores, so rather than improving the error I've fixed the bug in #2235.