perf skip isFile check during readBlock

feat: add N5NoSuchKeyException

src/main/java/org/janelia/saalfeldlab/n5/FileSystemKeyValueAccess.java

@@ -160,7 +160,11 @@ public FileSystemKeyValueAccess(final FileSystem fileSystem) {
 	@Override
 	public LockedFileChannel lockForReading(final String normalPath) throws IOException {
 
-		return new LockedFileChannel(normalPath, true);
+		try {
+			return new LockedFileChannel(normalPath, true);
+		} catch (NoSuchFileException e) {
+			throw new N5Exception.N5NoSuchKeyException("No such file", e);
+		}
 	}
 
 	@Override

src/main/java/org/janelia/saalfeldlab/n5/GsonKeyValueN5Reader.java

@@ -94,11 +94,11 @@ default DataBlock<?> readBlock(
 			final long... gridPosition) throws N5Exception {
 
 		final String path = absoluteDataBlockPath(N5URI.normalizeGroupPath(pathName), gridPosition);
-		if (!getKeyValueAccess().isFile(path))
-			return null;
 
 		try (final LockedChannel lockedChannel = getKeyValueAccess().lockForReading(path)) {
 			return DefaultBlockReader.readBlock(lockedChannel.newInputStream(), datasetAttributes, gridPosition);
+		} catch (final N5Exception.N5NoSuchKeyException e) {
+			return null;
 		} catch (final IOException | UncheckedIOException e) {
 			throw new N5IOException(
 					"Failed to read block " + Arrays.toString(gridPosition) + " from dataset " + path,

src/main/java/org/janelia/saalfeldlab/n5/N5Exception.java

@@ -94,4 +94,31 @@ protected N5ClassCastException(
 			super(message, cause, enableSuppression, writableStackTrace);
 		}
 	}
+
+	public static class N5NoSuchKeyException extends N5IOException {
+
+		public N5NoSuchKeyException(final String message) {
+
+			super(message);
+		}
+
+		public N5NoSuchKeyException(final String message, final Throwable cause) {
+
+			super(message, cause);
+		}
+
+		public N5NoSuchKeyException(final Throwable cause) {
+
+			super(cause);
+		}
+
+		protected N5NoSuchKeyException(
+				final String message,
+				final Throwable cause,
+				final boolean enableSuppression,
+				final boolean writableStackTrace) {
+
+			super(message, cause, enableSuppression, writableStackTrace);
+		}
+	}
 }