Check return of BlockDevice::init() in TDBStore.

Return value was ignored, and TDBStore:init() ended up in a
MBED_ERROR() phase after that.

TDBStore API was limited to allow returning of only two separate
errors, which may end up hiding the actual return value. Change
the documentation slightly to allow returning of original error
code from the underlying block device.

Fixes #11591

features/storage/kvstore/tdbstore/TDBStore.cpp

@@ -1019,7 +1019,10 @@ int TDBStore::init()
     _size = (size_t) -1;
 
     _buff_bd = new BufferedBlockDevice(_bd);
-    _buff_bd->init();
+    ret = _buff_bd->init();
+    if (ret) {
+        goto fail;
+    }
 
     // Underlying BD must have flash attributes, i.e. have an erase value
     if (_bd->get_erase_value() == -1) {
@@ -1140,6 +1143,9 @@ int TDBStore::init()
     _is_initialized = true;
     _mutex.unlock();
     return ret;
+fail:
+    mutex.unlock();
+    return ret;
 }
 
 int TDBStore::deinit()

features/storage/kvstore/tdbstore/TDBStore.h

@@ -61,8 +61,7 @@ class TDBStore : public KVStore {
      *        the available data and clean corrupted and erroneous records.
      *
      * @returns MBED_SUCCESS                        Success.
-     *          MBED_ERROR_READ_FAILED              Unable to read from media.
-     *          MBED_ERROR_WRITE_FAILED             Unable to write to media.
+     * @returns Negative error code on failure.
      */
     virtual int init();