throw loader exception on repeated string in StringPool.readFrom

[aizu-m]

Fed a hand-corrupted .xsb at the schema loader to watch where it fell over:

java.lang.IllegalStateException
    at org.apache.xmlbeans.impl.schema.SchemaTypeSystemImpl$StringPool.readFrom(SchemaTypeSystemImpl.java:728)
    at org.apache.xmlbeans.impl.schema.XsbReader.<init>(XsbReader.java:103)
    at org.apache.xmlbeans.impl.schema.SchemaTypeSystemImpl.resolveHandle(SchemaTypeSystemImpl.java:954)

readFrom fills the string pool by reading each entry and checking its assigned code equals the running index i. codeForString hands back the earlier code for a repeated string, so a pool that lists the same string twice makes code != i and trips a bare IllegalStateException with no message. readFrom only wraps IOException, so it leaks out as an undocumented runtime exception rather than the SchemaTypeLoaderException the loader is meant to raise; crackPointer catches only IOException too and sees the same thing while cracking a pointer file.

Threw SchemaTypeLoaderException the way stringForCode and the rest of the pool already report trouble. The size/emptiness check above it is a real internal invariant, so I left it. Valid files read back unchanged.