be-js: report an unconfigured library instead of throwing

[notactuallytreyanastasio]

temper run -b js looked the library up with getValue before
checking anything, so a --library that names no library, or one
whose config file does not parse, ended in an uncaught Kotlin
exception:

Exception in thread "main" java.util.NoSuchElementException: Key no-such-library is missing in the map.
    at lang.temper.be.js.RunJsKt.runJsBestEffort(RunJs.kt:69)

The metadata lookup right below it already returns a CliFailure for a
library the build does not know, as py, lua and java do. The
getValue threw first. A plain get lets that check run; the config
is only read for its library name, which is null-safe already.

Now:

## exception.msg: No JS library name for RunLibraryRequest(libraryName=no-such-library, taskName=run)
Run failed

RunTest.runJsBackendUnknownLibrary runs the run input with a
library name it does not have. Before this change it fails with the
NoSuchElementException; after, the run reports failure.

Fixes #611.

Reproduction: 58-js-run-missing-library-crash/ in temper-issue-repros.

🤖 Generated with Claude Code

[mikesamuel]

Failing over to the already established default path seems like a fine fix. Thanks for including a test.