Don't emit "javap not found in path" if it is available in the path but it did...

Don't emit "javap not found in path" if it is available in the path but it did not result in an actual difference.

diffoscope/comparators/java.py

@@ -34,15 +34,13 @@ logger = logging.getLogger(__name__)
 
 
 class ProcyonDecompiler(Command):
-    TOOL = "procyon"
-
     def __init__(self, path, *args, **kwargs):
         super().__init__(path, *args, **kwargs)
         self.real_path = os.path.realpath(path)
 
-    @tool_required(TOOL)
+    @tool_required("procyon")
     def cmdline(self):
-        return [self.TOOL, "-ec", self.path]
+        return ["procyon", "-ec", self.path]
 
     def filter(self, line):
         if re.match(r"^(//)", line.decode("utf-8")):
@@ -51,16 +49,14 @@ class ProcyonDecompiler(Command):
 
 
 class Javap(Command):
-    TOOL = "javap"
-
     def __init__(self, path, *args, **kwargs):
         super().__init__(path, *args, **kwargs)
         self.real_path = os.path.realpath(path)
 
-    @tool_required(TOOL)
+    @tool_required("javap")
     def cmdline(self):
         return [
-            self.TOOL,
+            "javap",
             "-verbose",
             "-constants",
             "-s",
@@ -86,6 +82,7 @@ class ClassFile(File):
 
     def compare_details(self, other, source=None):
         diff = []
+        last_exc = None
 
         for decompiler in self.decompilers:
             try:
@@ -95,12 +92,17 @@ class ClassFile(File):
                 if single_diff:
                     diff.append(single_diff)
                     break
-            except RequiredToolNotFound:
+            except RequiredToolNotFound as exc:
+                # Save our exception
+                last_exc = exc
                 logger.debug(
-                    "Unable to find %s. Falling back...", decompiler.TOOL,
+                    "Unable to find %s. Falling back...", decompiler,
                 )
 
-        if not diff:
-            raise RequiredToolNotFound(self.decompilers[-1].TOOL)
+        # Re-raise the last exception we would have raised from the previous
+        # loop; we want to raise the least-common demoninator from our
+        # `decompilers` list.
+        if last_exc:
+            raise last_exc
 
         return diff