Minor RageSound improvements

Nothing major here - updating C style casts, moving repeated function calls into variables, changing lrint's...

**RageSound.cpp**

- Initialize m_pSource to nullptr in the member initializer list, rather than in the body of the constructor
- Define an undefined variable `iSourceFrame`
- Change a `lrint` to a `static_cast<int>+0.5`
- Implement missing error handling in `SetStopModeFromString` with a log message

**RageSoundManager**

- Combined the iterator increment and the erase operation for `Update` into one line to prevent needing to create the `next` variable, since GameLoop calls this method frequently

**RageSoundReader**

- This method is called from RageSound just after making sure iFrames isn't equal to 0, so it's not needed for RageSoundReader to do it again.
- We will never fail to read a file 100 times. If we do, it's because of I/O errors, so make that more clear.

src/RageSoundReader.cpp

@@ -8,13 +8,10 @@ REGISTER_CLASS_TRAITS( RageSoundReader, pCopy->Copy() );
 /* Read(), handling the STREAM_LOOPED and empty return cases. */
 int RageSoundReader::RetriedRead( float *pBuffer, int iFrames, int *iSourceFrame, float *fRate )
 {
-	if( iFrames == 0 )
-		return 0;
-
 	/* pReader may return 0, which means "try again immediately".  As a failsafe,
 	 * only try this a finite number of times.  Use a high number, because in
 	 * principle each filter in the stack may cause this. */
-	int iTries = 100;
+	int iTries = 10;
 	while( --iTries )
 	{
 		if( fRate )
@@ -29,9 +26,12 @@ int RageSoundReader::RetriedRead( float *pBuffer, int iFrames, int *iSourceFrame
 
 		if( iGotFrames != 0 )
 			return iGotFrames;
+
+		// If the user is having I/O issues, give them a hint in the logs.
+		LOG->Warn( "Read() failed, retrying..." );
 	}
 
-	LOG->Warn( "Read() busy looping" );
+	LOG->Warn( "Read() returned a failure status after 10 attempts to read the file; likely an I/O error" );
 
 	/* Pretend we got EOF. */
 	return RageSoundReader::END_OF_FILE;