Replace usages of B_FILE_NOT_FOUND in public-facing APIs.

As BeOS R5 had deprecated this error code, this should in theory
not cause any particular problems.

src/kits/midi/SoftSynth.cpp

@@ -168,7 +168,7 @@ BSoftSynth::SetInstrumentsFile(const char* path)
 		return B_BAD_VALUE;
 	
 	if (!BEntry(path).Exists())
-		return B_FILE_NOT_FOUND;
+		return B_ENTRY_NOT_FOUND;
 
 	if (IsLoaded())
 		Unload();

src/system/runtime_loader/elf_versioning.cpp

@@ -181,7 +181,7 @@ check_needed_image_versions(image_t* image)
 			// the file should also appear in DT_NEEDED.
 			FATAL("%s: Version dependency \"%s\" not found", image->path,
 				fileName);
-			return B_FILE_NOT_FOUND;
+			return B_ENTRY_NOT_FOUND;
 		}
 
 		elf_vernaux* vernaux

src/tests/kits/storage/EntryTest.cpp

@@ -143,7 +143,6 @@ static const status_t kErrors[] = {
 	B_ERROR,
 	B_FILE_ERROR,
 	B_FILE_EXISTS,
-	B_FILE_NOT_FOUND,
 	B_IS_A_DIRECTORY,
 	B_LINK_LIMIT,
 	B_NAME_TOO_LONG,

src/tools/cppunit/TestUtils.cpp

@@ -35,10 +35,6 @@ status_t DecodeResult(status_t result) {
 			str = "B_FILE_ERROR";
 			break;
 
-		case B_FILE_NOT_FOUND:
-			str = "B_FILE_NOT_FOUND";
-			break;
-
 		case B_FILE_EXISTS:
 			str = "B_FILE_EXISTS";
 			break;