statprof: require paths to save or load profile data

Upstream appears to aggressively save statprof data in a well-defined
home directory path. Change the code to not do that.

We also change file saving to fail if an error has occurred
instead of silently failing. Callers can catch the exception.

This behavior is more suitable for a generic "library" module.

mercurial/statprof.py

@@ -311,13 +311,11 b' def stop():'
         state.accumulate_time(clock())
         state.last_start_time = None
         statprofpath = os.environ.get('STATPROF_DEST')
-        save_data(statprofpath)
+        if statprofpath:
+            save_data(statprofpath)
 
-def save_data(path=None):
-    try:
-        path = path or (os.environ['HOME'] + '/statprof.data')
-        file = open(path, "w+")
-
+def save_data(path):
+    with open(path, 'w+') as file:
         file.write(str(state.accumulated_time) + '\n')
         for sample in state.samples:
             time = str(sample.time)
@@ -326,13 +324,7 b' def save_data(path=None):'
                      for s in stack]
             file.write(time + '\0' + '\0'.join(sites) + '\n')
 
-        file.close()
-    except (IOError, OSError):
-        # The home directory probably didn't exist, or wasn't writable. Oh well.
-        pass
-
-def load_data(path=None):
-    path = path or (os.environ['HOME'] + '/statprof.data')
+def load_data(path):
     lines = open(path, 'r').read().splitlines()
 
     state.accumulated_time = float(lines[0])