MQE-1254: MFTF with Firefox fails due to getLog call

[aljcalandra]

• Added check to ascertain browser is among the available log types

Prevents a call to getLog for a type of log that may not exist

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/verification tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)
• Changes to Framework doesn't have backward incompatible changes for tests or have related Pull Request with fixes to tests

[jilu1]

Looks good

[coveralls]

Coverage remained the same at 57.398% when pulling ea9693d on MQE-1254 into 66b6ecd on develop.