Don't err with the "string out of bounds" exception while throwing "Unsupported command" exception

Out of the box agpdiff failed on my SQL dump with:

    Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 85
        at java.lang.String.substring(String.java:1934)
        at cz.startnet.utils.pgdiff.parsers.Parser.throwUnsupportedCommand(Parser.java:390)
        at cz.startnet.utils.pgdiff.parsers.AlterTableParser.parseAlterColumn(AlterTableParser.java:291)
        at cz.startnet.utils.pgdiff.parsers.AlterTableParser.parse(AlterTableParser.java:79)
        at cz.startnet.utils.pgdiff.loader.PgDumpLoader.loadDatabaseSchema(PgDumpLoader.java:180)
        at cz.startnet.utils.pgdiff.loader.PgDumpLoader.loadDatabaseSchema(PgDumpLoader.java:236)
        at cz.startnet.utils.pgdiff.PgDiff.createDiff(PgDiff.java:29)
        at cz.startnet.utils.pgdiff.Main.main(Main.java:39)

With this patch it fails with a more sensible:

    Exception in thread "main" cz.startnet.utils.pgdiff.parsers.ParserException: Cannot parse string: ALTER TABLE download_texts ALTER COLUMN download_text_length set NOT NULL;
    Unsupported command at position 66 'NOT NULL;'
        at cz.startnet.utils.pgdiff.parsers.Parser.throwUnsupportedCommand(Parser.java:390)
        at cz.startnet.utils.pgdiff.parsers.AlterTableParser.parseAlterColumn(AlterTableParser.java:291)
        at cz.startnet.utils.pgdiff.parsers.AlterTableParser.parse(AlterTableParser.java:79)
        at cz.startnet.utils.pgdiff.loader.PgDumpLoader.loadDatabaseSchema(PgDumpLoader.java:180)
        at cz.startnet.utils.pgdiff.loader.PgDumpLoader.loadDatabaseSchema(PgDumpLoader.java:236)
        at cz.startnet.utils.pgdiff.PgDiff.createDiff(PgDiff.java:29)
        at cz.startnet.utils.pgdiff.Main.main(Main.java:39)