[#1267] Avoid Division By Zero in ScanProgress type

Although is not the best way to solve it this addresses the issue
in a single statement and following existing pattern in the code.

The error should be thrown earlier in RustBackend itself if so,
or fallback to some specific value that makes sense to the domain

Closes #1267

Sources/ZcashLightClientKit/Model/ScanProgress.swift

@@ -12,8 +12,11 @@ struct ScanProgress: Equatable {
     let denominator: UInt64
     
     func progress() throws -> Float {
-        // division by 0 is not a concern here because `ZcashRustBackend.getScanProgress() -> ScanProgress?`
-        // handles the 0 and returns nil rather than returning nil progress or 0 value here
+        guard denominator != 0 else {
+            // this shouldn't happen but if it does, we need to get notified by clients and work on a fix
+            throw ZcashError.rustScanProgressOutOfRange("\(numerator)/\(denominator)")
+        }
+
         let value = Float(numerator) / Float(denominator)
         
         // this shouldn't happen but if it does, we need to get notified by clients and work on a fix

Tests/OfflineTests/ZcashRustBackendTests.swift

@@ -157,4 +157,14 @@ class ZcashRustBackendTests: XCTestCase {
         XCTAssertEqual(metadata?.networkType, .mainnet)
         XCTAssertEqual(metadata?.addressType, .sapling)
     }
+
+    func testScanProgressThrowsOnWrongValues() {
+        // Assert that throws on Zero denominator
+        XCTAssertThrowsError(try ScanProgress(numerator: 0, denominator: 0).progress())
+
+        // Assert that throws on numerator > denominator
+        XCTAssertThrowsError(try ScanProgress(numerator: 23, denominator: 2).progress())
+
+        XCTAssertNoThrow(try ScanProgress(numerator: 3, denominator: 4).progress())
+    }
 }