We recently changed the way that we check whether babel, lessc, rollup

and uglifyjs binaries exist in the node modules path when building

Pipeline settings. We used to hard-code paths like

<node_modules_path>/.bin/rollup and check for their existence, but now

we use the npm exec command.
However, we invoke the command from the current working directory, when we

should actually be invoking it from the parent directory of the given node

modules path. This was made evident when trying to build Power Pack or run

its unit tests. Power Pack relies on the binaries in Review Board's node

modules, but npm exec was being run from Power Pack's node modules

directory.
This change fixes this so that the binaries are actually checked for in

the node modules paths, instead of the current working directory. And

we now include the consuming application's node modules path when

building the static media for an extension. If the binary is found,

then we use that path when running the binary with npm exec.
We also add handling for when node_modules_path is an empty string.

This can happen if we didn't find any node modules directories, which

should be an error state. When validate_paths=False and we have an

empty node_modules_path, we use the current working directory as

the path.
And finally this updates the docstring to (I hope) clarify the

validate_paths argument and when it should be set to False, and

clarifies the expectations of node_modules_path when

validate_paths=False.

Ran unit tests.
Was able to run Power Pack unit tests without hitting the

ImproperlyConfigured error for rollup.
Was able to build a Power Pack package against a development server

  of Review Board without hitting the ImproperlyConfigured error for

  rollup.
Set node_modules_path with validate_paths=True to an empty string,

  a real path valid path and a non-existent one separated by a colon,

  a real path valid path, and a non-existent path.
Ran the dev server, previously saw an error about the rollup command

  failing but its gone away with this fix.