Fix exception causes all over the codebase

gitdb/__init__.py

@@ -18,8 +18,8 @@ def _init_externals():
 
         try:
             __import__(module)
-        except ImportError:
-            raise ImportError("'%s' could not be imported, assure it is located in your PYTHONPATH" % module)
+        except ImportError as e:
+            raise ImportError("'%s' could not be imported, assure it is located in your PYTHONPATH" % module) from e
         # END verify import
     # END handel imports
 

gitdb/db/mem.py

@@ -74,8 +74,8 @@ def stream(self, sha):
             # rewind stream for the next one to read
             ostream.stream.seek(0)
             return ostream
-        except KeyError:
-            raise BadObject(sha)
+        except KeyError as e:
+            raise BadObject(sha) from e
         # END exception handling
 
     def size(self):

gitdb/util.py

@@ -326,8 +326,8 @@ def open(self, write=False, stream=False):
             else:
                 self._fd = fd
             # END handle file descriptor
-        except OSError:
-            raise IOError("Lock at %r could not be obtained" % self._lockfilepath())
+        except OSError as e:
+            raise IOError("Lock at %r could not be obtained" % self._lockfilepath()) from e
         # END handle lock retrieval
 
         # open actual file if required