Handle no-search-hits properly

Before this change, if there were no search hits, we just listed all
running processes.

With this change in place, we print an informative message to stderr
when this happens.

Fixes #117.

px/px_tree.py

@@ -1,16 +1,26 @@
 from . import px_process
 from . import px_terminal
 
+import sys
+
 from typing import Set, List, Optional, Iterable
 
 
 def tree(search: str) -> None:
     """Print a process tree"""
-    for line in _generate_tree(px_process.get_all(), search):
+    print_me = _generate_tree(px_process.get_all(), search)
+    if not print_me:
+        print(f"No processes found matching <{search}>", file=sys.stderr)
+        return
+
+    for line in print_me:
         print(line)
 
 
 def _generate_tree(processes: List[px_process.PxProcess], search: str) -> List[str]:
+    if not processes:
+        return []
+
     # Only print subtrees needed for showing all search hits and their children.
     #
     # We do that by starting at the search hits and walking up the tree from all
@@ -24,6 +34,7 @@ def _generate_tree(processes: List[px_process.PxProcess], search: str) -> List[s
 
             _mark_children(process, show_pids)
 
+            # Walk up the tree from this process, marking each process for display
             if process and process.parent:
                 process = process.parent
             while process:
@@ -34,8 +45,9 @@ def _generate_tree(processes: List[px_process.PxProcess], search: str) -> List[s
                     break
                 process = process.parent
 
-    if not processes:
-        return []
+        # Search found nothing
+        if not show_pids:
+            return []
 
     lines = []
     coalescer = Coalescer(0)

tests/px_tree_test.py

@@ -47,6 +47,37 @@ def test_search():
     ) == ["root(1)", f"  {px_terminal.bold('find-me')}(2)"]
 
 
+def test_no_search():
+    assert px_tree._generate_tree(
+        resolve(
+            [
+                testutils.create_process(pid=1, ppid=0, commandline="root"),
+                testutils.create_process(pid=2, ppid=1, commandline="find-me"),
+                testutils.create_process(pid=3, ppid=1, commandline="uninteresting"),
+            ]
+        ),
+        "",
+    ) == ["root(1)", "  find-me(2)", "  uninteresting(3)"]
+
+
+def test_search_no_matches():
+    assert (
+        px_tree._generate_tree(
+            resolve(
+                [
+                    testutils.create_process(pid=1, ppid=0, commandline="root"),
+                    testutils.create_process(pid=2, ppid=1, commandline="find-me"),
+                    testutils.create_process(
+                        pid=3, ppid=1, commandline="uninteresting"
+                    ),
+                ]
+            ),
+            "not-there",
+        )
+        == []
+    )
+
+
 def test_coalesce_no_search():
     assert px_tree._generate_tree(
         resolve(