The plug-in manifest is validated in two phases: i) when it is first loaded,
ii) when the manifest is inline edited. In the former case, the validation
is carried out in OSGI layers, and the exception caught in the pde modules
with no bearing on the faulty line number, in case of issues.

This commit tries to parse the exception message in a best effort basis,
extracts the header that is subjected for the fault and obtains the line
number of the header. It falls back on line 1, in case of parse error.