Incorporate helpful error messages when MASTER is unable to generate newick/nexus trees

[gtng92]

To prevent this message:

> obs.tree <- epidem.model(theta, nsim=1, tips=100, tsample=0.1, model='epidemic')[[1]]
Warning message:
In read.tree(file = "temp.newick") : empty character string.

[ArtPoon]

A check for this can be written at the end of master_xml_template.R:.call_master()

[gtng92]

Issue #120 There may be other checks that need to be incorporated in the future.