Close the .xsb output when a save is abandoned part-way through

[pjfanning]

Every saveXxx() in SchemaTypeSystemImpl (saveIndex, savePointerFile, saveGlobalElement, saveGlobalAttribute, saveModelGroup, saveAttributeGroup, saveIdentityConstraint, saveType) runs the same sequence:

XsbReader saver = new XsbReader(getTypeSystem(), handle);
saver.writeTypeData(type);                            // fills the string pool
saver.writeRealHeader(handle, FILETYPE_SCHEMATYPE);   // opens the Filer output stream
saver.writeTypeData(type);                            // the real write
saver.writeEnd();                                     // flush + close

Nothing guards the middle. Every write helper raises SchemaTypeLoaderException on an IOException, so a failing write skips writeEnd() and leaves the .xsb output stream open over a partial file. scomp writes one file per global type, so a failure part-way through a large schema leaks a descriptor for each one already opened.

Adds XsbReader.closeOutputQuietly() — a no-op once writeEnd() has cleared _output — and calls it from a finally in all eight save methods. Closing quietly rather than calling writeEnd() keeps the original failure from being masked by a second SchemaTypeLoaderException.

Added XsbSaveStreamTest, which serves a Filer output stream that opens and then fails every write, and asserts the stream is closed. It fails on trunk and passes with this change. compile.scomp.checkin.CompilationTests still passes.

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