Tighten the exception SPVBlockStore throws when the chain file is locked.

core/src/main/java/org/bitcoinj/store/ChainFileLockedException.java

@@ -0,0 +1,14 @@
+package org.bitcoinj.store;
+
+/**
+ * Thrown by {@link SPVBlockStore} when the process cannot gain exclusive access to the chain file.
+ */
+public class ChainFileLockedException extends BlockStoreException {
+    public ChainFileLockedException(String message) {
+        super(message);
+    }
+
+    public ChainFileLockedException(Throwable t) {
+        super(t);
+    }
+}

core/src/main/java/org/bitcoinj/store/SPVBlockStore.java

@@ -103,7 +103,7 @@ public class SPVBlockStore implements BlockStore {
             FileChannel channel = randomAccessFile.getChannel();
             fileLock = channel.tryLock();
             if (fileLock == null)
-                throw new BlockStoreException("Store file is already locked by another process");
+                throw new ChainFileLockedException("Store file is already locked by another process");
 
             // Map it into memory read/write. The kernel will take care of flushing writes to disk at the most
             // efficient times, which may mean that until the map is deallocated the data on disk is randomly