extra/runner/report.py: Include "report.py" in error messages

There is also a shell script called `report` (which calls `report.py`)
so this change clarifies where the error message is coming from.
extra/runner/report: Don't resolve symlinks when determining results directory

The directory structure of a "test results" directory (where you ran
`extra/runner/run` from) looks like this:

    results/
    results/2013-08-10_09.54.29/
    results/2013-08-10_09.56.06/
    results/index.html

However, a user can manually create such a structure using symlinks
instead, when they want to create a report (using `extra/runner/report`)
that contains results from several different "test results" directories:

    summary/
    summary/2013-08-10_09.54.29-run1 -> ../results1/2013-08-10_09.54.29
    summary/2013-08-10_09.56.06-run1 -> ../results1/2013-08-10_09.56.06
    etc.

In that case, running `report summary/2013*` would create the summary
report in `results1/index.html` instead of `summary/index.html`. This
commit fixes that; there is no need to resolve symlinks when determining
the "test results" directory.

This uses python's `abspath` (which doesn't resolve symlinks) instead of
python's `realpath`. Since we *do* need to resolve symlinks elsewhere in
the `report` script†, we need both functions. The `run` script only uses
python's `realpath`, but I've renamed `run`'s shell function to
`realpath` for consistency with the python function name and for
consistency with `report`.

† To find the directory containing the `report.py` script (which is the
same directory that contains this `report` shell script). Some people
symlink `report` into a directory on their PATH (such as `~/bin`)
instead of adding the directory containing `report` to their PATH.