Trap currentIssue in browseVolume.ftl

[rcave]

If the current issue isn't defined for a journal, it shouldn't throw an error:

2009-04-28 12:20:04,196 ERROR runtime(PLoSONE)>  [http-8080-Processor24 freemarker.runtime]
Expression currentIssue is undefined on line 31, column 26 in journals/plosone/article/browseVolume.ftl.
The problematic instruction:
----------
==> ${currentIssue.id} [on line 31, column 24 in journals/plosone/article/browseVolume.ftl]
 in user-directive s.url [on line 30, column 9 in journals/plosone/article/browseVolume.ftl]
 in include "${templateFile}" [on line 31, column 1 in templates/main_template.ftl]
----------

Java backtrace for programmers:
----------
freemarker.core.InvalidReferenceException: Expression currentIssue is undefined on line 31, column 26 in journals/plosone/a
rticle/browseVolume.ftl.
        at freemarker.core.TemplateObject.assertNonNull(TemplateObject.java:124)
        at freemarker.core.TemplateObject.invalidTypeException(TemplateObject.java:134)
...
...
2009-04-28 12:20:04,199 ERROR runtime(PLoSONE)>  [http-8080-Processor24 freemarker.runtime]
Expression currentIssue is undefined on line 31, column 26 in journals/plosone/article/browseVolume.ftl.
The problematic instruction:
----------
==> ${currentIssue.id} [on line 31, column 24 in journals/plosone/article/browseVolume.ftl]
 in user-directive s.url [on line 30, column 9 in journals/plosone/article/browseVolume.ftl]
 in include "${templateFile}" [on line 31, column 1 in templates/main_template.ftl]
----------

Java backtrace for programmers:
----------
freemarker.core.InvalidReferenceException: Expression currentIssue is undefined on line 31, column 26 in journals/plosone/a
rticle/browseVolume.ftl.
        at freemarker.core.TemplateObject.assertNonNull(TemplateObject.java:124)
        at freemarker.core.TemplateObject.invalidTypeException(TemplateObject.java:134)
...
...

[josowski]

(In [7630]) If currentIssue is undefined, this page would error out. This resolves that issue.

References #1275

[rcave]

Verified that currentIssue is trapped. Added to the plosone browseVolume.ftl.