stbt.get_config: Handle all ConfigParser errors
stbt.get_config: Handle all ConfigParser errors

...and use ConfigParser's own error messages:

* No option 'no_such_key' in section: 'global'
* No section: 'no_such_section'
etc.

Note that ConfigParser supports interpolation, so values can refer to
other values, e.g.

    [My Section]
    foodir: %(dir)s/whatever
    dir=frob

so there are many possible ConfigParser exception types other than
NoSectionError and NoOptionError.

wmanley/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester