Add failable persistent cache initializer

Sources/SPTPersistentCache.m

@@ -79,6 +79,11 @@ - (instancetype)init
     return [self initWithOptions:[SPTPersistentCacheOptions new]];
 }
 
+- (nullable instancetype)initFailableWithOptions:(SPTPersistentCacheOptions *)options
+{
+    return [self initWithOptions:options];
+}
+
 - (instancetype)initWithOptions:(SPTPersistentCacheOptions *)options
 {
     self = [super init];

Tests/SPTPersistentCacheTests.m

@@ -1270,7 +1270,7 @@ - (void)testInitNilWhenCannotCreateCacheDirectory
         return nil;
     });
     method_setImplementation(originalMethod, fakeMethodImplementation);
-    SPTPersistentCache *cache = [[SPTPersistentCache alloc] initWithOptions:options];
+    SPTPersistentCache *cache = [[SPTPersistentCache alloc] initFailableWithOptions:options];
     method_setImplementation(originalMethod, originalMethodImplementation);
 
     XCTAssertNil(cache, @"The cache should be nil if it could not create the directory");

include/SPTPersistentCache/SPTPersistentCacheImplementation.h

@@ -107,6 +107,13 @@ typedef NSString * _Nonnull(^SPTPersistentCacheChooseKeyCallback)(NSArray<NSStri
  */
 - (instancetype)initWithOptions:(SPTPersistentCacheOptions *)options NS_DESIGNATED_INITIALIZER;
 
+/**
+ Failable initialiser.
+ @param options The options to use for the cache parameters.
+ @return An initialised cache, or `nil` if the cache directory cannot be created.
+ */
+- (nullable instancetype)initFailableWithOptions:(SPTPersistentCacheOptions *)options;
+
 /**
  @discussion Load data from cache for specified key.
  Req.#1.2. Expired records treated as not found on load. (And open stream)