Correctly attribute exceptions thrown inside a Worker's imported scripts.

LayoutTests/platform/win/http/tests/workers/worker-importScripts-expected.txt

-Test importScripts.
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-Test started.
-PASS: importScripts(), exists, is a function, and doesn't throw when not given any arguments
-Loaded message from different origin
-PASS: executed script from different origin
-Loaded message from different origin
-PASS: executed script from different origin through a redirect
-Testing single argument:
-Loaded resource 1
-PASS: loaded first source
-Testing multiple arguments:
-Loaded resource 1
-Loaded resource 2
-PASS: Both sources loaded and executed.
-Testing multiple arguments (different order):
-Resource 2 loaded before resource 1
-Loaded resource 2
-Loaded resource 1
-PASS: Both sources loaded and executed.
-Testing multiple arguments, with different origin for one argument:
-Loaded resource 1
-Loaded message from different origin
-Loaded resource 2
-PASS: all resources executed.
-Loaded resource 1
-PASS: Threw NetworkError: A network error occurred. when load failed
-PASS: some resources were loaded despite the network error
-Loaded resource 1
-PASS: Threw SyntaxError: Unexpected identifier when encountering a syntax error in imported script
-PASS: some resources were loaded despite the presence of a syntax error
-Testing multiple arguments, with first resource throwing an exception:
-Loaded resource 1
-First resource throwing an exception
-PASS: Propagated 'Thrown by first resource' from script
-PASS: First resource was executed, and second resource was not
-Testing multiple arguments, with second resource throwing an exception:
-Loaded resource 1
-Loaded resource 2
-Second resource throwing an exception
-PASS: Propagated 'Thrown by second resource' from script
-PASS: Both scripts were executed
-DONE
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