Bug 1393503 - Proper error handling for failure to find VCS tool; r=glandium,nalexander

``print()`` has no business being in library code like this. It was
a holdover from this code being copied from bootstrap. So remove it.

While we're here, replace the generic exception with a specific one.
We don't want to be swallowing bugs via ``except Exception``.

MozReview-Commit-ID: 49goUstfPBz

--HG--
extra : rebase_source : a821159bd12a449ed1a0edf21a1f9eb29711ad95

build/mach_bootstrap.py

@@ -169,11 +169,8 @@ def bootstrap(topsrcdir, mozilla_dir=None):
             # If we ever need to use the VCS binary here, consider something
             # more robust.
             return mozversioncontrol.get_repository_object(path=mozilla_dir)
-        except mozversioncontrol.InvalidRepoPath:
-            return None
-        # This is mainly to catch failures resolving the VCS binary path.
-        # TODO Change mozversioncontrol to raise non-generic exception.
-        except Exception:
+        except (mozversioncontrol.InvalidRepoPath,
+                mozversioncontrol.MissingVCSTool):
             return None
 
     def telemetry_handler(context, data):

python/mozversioncontrol/mozversioncontrol/__init__.py

@@ -14,6 +14,10 @@ import which
 from distutils.version import LooseVersion
 
 
+class MissingVCSTool(Exception):
+    """Represents a failure to find a version control tool binary."""
+
+
 def get_tool_path(tool):
     """Obtain the path of `tool`."""
     if os.path.isabs(tool) and os.path.exists(tool):
@@ -30,11 +34,11 @@ def get_tool_path(tool):
         try:
             return which.which(tool)
         except which.WhichError as e:
-            print(e)
+            pass
 
-    raise Exception('Unable to obtain %s path. Try running '
-                    '|mach bootstrap| to ensure your environment is up to '
-                    'date.' % tool)
+    raise MissingVCSTool('Unable to obtain %s path. Try running '
+                         '|mach bootstrap| to ensure your environment is up to '
+                         'date.' % tool)
 
 
 class Repository(object):