wic: refactored processing of wic exceptions

All wic exceptions are now inherited from new base exception
class WicError. It makes them easy to maintain and catch.

Processing of exceptions is done this way:
    Known wic exceptions cause wic to print error message to stdout.
    Unknown exceptions are not catched anymore and produce standard
    python traceback.

(From OE-Core rev: e5e2c18ce4344c14d9e52ece916333bd0a619281)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/wic

@@ -53,6 +53,7 @@ else:
     bitbake_main = None
 
 from wic.utils.oe.misc import find_bitbake_env_lines, set_bitbake_env_lines
+from wic.utils.errors import WicError
 from image import engine
 from image import help as hlp
 
@@ -309,10 +310,8 @@ def main(argv):
 
 if __name__ == "__main__":
     try:
-        ret = main(sys.argv[1:])
-    except Exception:
-        ret = 1
-        import traceback
-        traceback.print_exc()
-    sys.exit(ret)
+        sys.exit(main(sys.argv[1:]))
+    except WicError as err:
+        print >> sys.stderr, "ERROR:", err
+        sys.exit(1)