exceptions: Classify transient exceptions

Exceptions such as TargetError can sometimes be raised because of a
network issue, which is useful to distinguish from errors caused by a
missing feature for automated testing environments.

The following exceptions are introduced:
* DevlibStableError: raised when a non-transient error is encountered
    * TargetStableError

* DevlibTransientError: raised when a transient error is encountered,
including timeouts.
    * TargetTransientError

When there is an ambiguity on the type of exception to use, it can be
assumed that the configuration is correct, and therefore it is a
transient error, unless the function is specifically designed to probe a
property of the system. In that case, ambiguity is allowed to be lifted
by assuming a non-transient error, since we expect it to raise an
exception when that property is not met. Such ambiguous case can appear
when checking Android has booted, since we cannot know if this is a
timeout/connection issue, or an actual issue with the Android build or
configuration. Another case are the execute() methods, which can be
expected to fail on purpose. A new parameter will_succeed=False is
added, to automatically turn non transient errors into transient ones if
the caller is 100% sure that the command cannot fail unless there is an
environment issue that is outside of the scope controlled by the user.

devlib now never raises TargetError directly, but one of
TargetStableError or TargetTransientError. External code can therefore
rely on all (indirect) instances TargetError to be in either category.
Most existing uses of TargetError are replaced by TargetStableError.

devlib/module/cgroups.py

@@ -18,7 +18,7 @@ import re
 from collections import namedtuple
 
 from devlib.module import Module
-from devlib.exception import TargetError
+from devlib.exception import TargetStableError
 from devlib.utils.misc import list_to_ranges, isiterable
 from devlib.utils.types import boolean
 
@@ -281,7 +281,7 @@ class CGroup(object):
             self.target.execute('[ -d {0} ]'\
                 .format(self.directory), as_root=True)
             return True
-        except TargetError:
+        except TargetStableError:
             return False
 
     def get(self):
@@ -319,7 +319,7 @@ class CGroup(object):
             # Set the attribute value
             try:
                 self.target.write_value(path, attrs[idx])
-            except TargetError:
+            except TargetStableError:
                 # Check if the error is due to a non-existing attribute
                 attrs = self.get()
                 if idx not in attrs:
@@ -389,9 +389,9 @@ class CgroupsModule(Module):
             controller = Controller(ss.name, hid, hierarchy[hid])
             try:
                 controller.mount(self.target, self.cgroup_root)
-            except TargetError:
+            except TargetStableError:
                 message = 'Failed to mount "{}" controller'
-                raise TargetError(message.format(controller.kind))
+                raise TargetStableError(message.format(controller.kind))
             self.logger.info('  %-12s : %s', controller.kind,
                              controller.mount_point)
             self.controllers[ss.name] = controller