oeqa: add proper handling for command errors where needed

For use outside of tests themselves, we want a better error than
AssertionError, so create one and allow us to request it when calling
runCmd(). This enables us to avoid tracebacks during master image
operations if the power control command fails.

(From OE-Core rev: 89868383685091b0d3723fb8f29590f3f6610078)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oeqa/utils/__init__.py

@@ -1,3 +1,15 @@
 # Enable other layers to have modules in the same named directory
 from pkgutil import extend_path
 __path__ = extend_path(__path__, __name__)
+
+
+# Borrowed from CalledProcessError
+
+class CommandError(Exception):
+    def __init__(self, retcode, cmd, output = None):
+        self.retcode = retcode
+        self.cmd = cmd
+        self.output = output
+    def __str__(self):
+        return "Command '%s' returned non-zero exit status %d with output: %s" % (self.cmd, self.retcode, self.output)
+