[mathiasrw]

Closes #822

Stacked on #827.

Change

--js code can throw anything, not only an error. The message is now made in a way that can not fail:

Code	Before	Now
throw Object.create(null)	the error handling crashed, with a stack trace and exit code 1	an object was thrown
throw {code: 42}	[object Object]	an object was thrown: {"code":42}
throw ""	an empty message	an empty string was thrown
throw new Error("x")	Error: x	the same

Tests

test/fix/822.test.js is new, and 3 of its 4 tests fail without the fix. yarn test-ci passes.

From the review of 23 October.

After the review of 24 October

• An error whose message can not be read is told by its name, like a TypeError was thrown.
• A thrown object is cut after 300 chars, as it can hold a whole file.

After the review of 24 October

Closes #845

• An object of a class of the user is told by its class and what it holds, like a Foo was thrown: {"a":1}. Before the last fix it lost the class, and after it, what it held.
• The whole message is cut after 300 characters, by characters and not UTF-16 units, so an emoji is not split into �. That covers a long class name too.
• "an" in front of a vowel: an Error was thrown, an Item was thrown.