Crypto: Check for error return from sign()

...g/java/src/net/i2p/client/streaming/impl/PacketLocal.java

core/java/src/net/i2p/crypto/DSAEngine.java

core/java/src/net/i2p/crypto/KeyGenerator.java

core/java/src/net/i2p/data/DatabaseEntry.java

core/java/src/net/i2p/data/PrivateKeyFile.java

core/java/src/net/i2p/data/i2cp/SessionConfig.java