UTIL_getFileSize() returns UTIL_FILESIZE_UNKNOWN on failure

UTIL_getFileSize() used to return zero on failure.
This made it impossible to distinguish a failure from a genuine empty file.
Both cases where coalesced.

Adding UTIL_FILESIZE_UNKNOWN constant has many consequences on user code,
since in many places, the `0` was assumed to mean "error".
This is no longer the case, and the error code must be actively checked.

tests/fullbench.c

@@ -484,8 +484,8 @@ static int benchFiles(const char** fileNamesTable, const int nbFiles, U32 benchN
     /* Loop for each file */
     int fileIdx;
     for (fileIdx=0; fileIdx<nbFiles; fileIdx++) {
-        const char* inFileName = fileNamesTable[fileIdx];
-        FILE* inFile = fopen( inFileName, "rb" );
+        const char* const inFileName = fileNamesTable[fileIdx];
+        FILE* const inFile = fopen( inFileName, "rb" );
         U64   inFileSize;
         size_t benchedSize;
         void* origBuff;
@@ -495,6 +495,11 @@ static int benchFiles(const char** fileNamesTable, const int nbFiles, U32 benchN
 
         /* Memory allocation & restrictions */
         inFileSize = UTIL_getFileSize(inFileName);
+        if (inFileSize == UTIL_FILESIZE_UNKNOWN) {
+            DISPLAY( "Cannot measure size of %s\n", inFileName);
+            fclose(inFile);
+            return 11;
+        }
         benchedSize = BMK_findMaxMem(inFileSize*3) / 3;
         if ((U64)benchedSize > inFileSize) benchedSize = (size_t)inFileSize;
         if (benchedSize < inFileSize)