stbt-run: Exit with different exit codes by exception type

To make interpreting test results by a test harness easier, `stbt run`
exits with exit code '0' on success, '1' if 'stbt.UITestError' is raised
indicating a test failure, and '2' if any other exception is raised
indicating a test case error or environment error.

The test execution framework shall be configured to report the two
types of failures differently, so that it's easy to distinguish between
bugs caught and other kinds of errors.
README: Document exit status
Merge branch 'different-exit-codes-by-exception-type'

* different-exit-codes-by-exception-type:
  README: Document exit status
  stbt-run: Exit with different exit codes by exception type

Closes #10.
stbt templatematch: Backwards compatibility support for `noise_threshold`

In 18a5e70e `noise_threshold` was renamed to `confirm_threshold`.

In 25d7f3a9 `confirm_threshold` was moved into a field of
`MatchParameters`, and `stbt templatematch` was updated accordingly, but
in a way that lost backwards compatibility support for
`noise_threshold`.

We'll keep `noise_threshold` for a release or two, and then we'll remove
it.
README: Update to match changes from 659f69c1

This commit is the result of running `make doc`, which keeps README.rst
in sync with the reality of the default values in the python code.
stbt.conf: Move image-matching parameters to [match] section

(From the [global] section.)

This doesn't break backwards compatibility because these parameters
(match_method, match_threshold, confirm_method, confirm_threshold,
erode_passes) were introduced (in dd44303b) more recently than the last
stb-tester release (0.12).

The need for a separate section is that in future we may want to put the
parameters for *motion* detection into stbt.conf, and those (and other
new) parameters may conflict with the image-matching parameters.
stbt.MatchParameters: Document the config file keys for default values
stbt tv: Search for `source_pipeline` in the correct config section

Since commit 9283df1f, `source_pipeline` is specified in the `[global]`
section of stbt.conf.
rpm spec file: Bump stb-tester version to 0.13