Move failWithErrorReason to a common place

This is a shared public utility. Its main utility is for methods
that can fail, have an error message of their own, and also want
to take an error message from a caller and want to combine them.
Example use :
fun <T> assertEquals(expected: T, actual: T, msg: String? = null) {
  if (expected != null) {
    failWithErrorReason(msg, "Expected $expected, got $actual")
  }
}
...which allows someone to say
assertEquals(a, b, "Checking a vs b")

Test: TH
Flag: EXEMPT test-only change
Change-Id: Id3eaa8384b4177f38f13576d9b4d742c52027ae4

staticlibs/testutils/devicetests/com/android/testutils/TestableNetworkCallback.kt

@@ -70,7 +70,7 @@ open class RecorderCallback private constructor(
         data class Reserved private constructor(
                 override val network: Network,
                 val caps: NetworkCapabilities
-        ): CallbackEntry() {
+        ) : CallbackEntry() {
             constructor(caps: NetworkCapabilities) : this(NULL_NETWORK, caps)
         }
         data class Available(override val network: Network) : CallbackEntry()
@@ -399,12 +399,6 @@ open class TestableNetworkCallback private constructor(
                 }
             } as T
 
-    // "Nothing" is the return type to declare a function never returns a value.
-    fun failWithErrorReason(errorMsg: String?, errorReason: String): Nothing {
-        val message = if (errorMsg != null) "$errorMsg : $errorReason" else errorReason
-        fail(message)
-    }
-
     inline fun <reified T : CallbackEntry> expect(
         network: HasNetwork,
         timeoutMs: Long = defaultTimeoutMs,

staticlibs/testutils/devicetests/com/android/testutils/TestableNetworkOfferCallback.kt

@@ -56,12 +56,6 @@ class TestableNetworkOfferCallback(
         history.add(CallbackEntry.Unneeded(request))
     }
 
-    // TODO : move to some utility, this is copied from TestableNetworkCallback
-    fun failWithErrorReason(errorMsg: String?, errorReason: String): Nothing {
-        val message = if (errorMsg != null) "$errorMsg : $errorReason" else errorReason
-        fail(message)
-    }
-
     inline fun <reified T : CallbackEntry> expect(
         errorMsg: String? = null,
         predicate: (T) -> Boolean = { true }

staticlibs/testutils/hostdevice/com/android/testutils/MiscAsserts.kt

@@ -142,4 +142,12 @@ fun assertEventuallyTrue(
         }
         Thread.sleep(pollIntervalMs)
     }
-}
+}
+
+// "Nothing" is the return type to declare a function never returns a value. This is useful
+// because the compiler will know the branch of the code calling this does not return, which
+// lets check exhaustivity, infer return types, nullability and smart casts.
+fun failWithErrorReason(errorMsg: String?, errorReason: String): Nothing {
+    val message = if (errorMsg != null) "$errorMsg : $errorReason" else errorReason
+    fail(message)
+}