The pyflakes tool wasn't handling all types of output correctly. Syntax

errors and unknown errors reported by pyflakes would trigger a crash,

which we would occasionally see in some uploaded code. There were also

issues where source code could manifest as errors that couldn't be

resolved.
This change redoes the processing to handle the various forms of output.

Pyflakes sends lintian errors to stdout and syntax/unknown errors to

stderr, in different formats.
Lintian errors are one line of content (the filename, line number, and

message).
Unknown errors are one line of content (the filename and message).
Syntax errors will show an info line (filename, line number, possible

offset, and message), a line of code, and a possible offset indicator

line (a ^ under the offending character).
Now that we can detect each type, we can make smarter assumptions about

what we're looking at. The syntax error case is the most complicated,

but it's parsable now with the new code.
To separate out stdout and stderr, execute() had to be updated to take

a return_errors option. This logic exactly matches the equivalent in

RBTools (which we should probably just wrap in a future change).
To ensure everything works as expected, there's now unit tests for

pyflakes that throws the various forms of output at the parser.

Unit tests pass.
This needs some manual testing in production.