Errors in labels crash during `graph`, are handled properly during `text` or in the legend strings

[esternin]

text `<i1>a</i0>b'

is an invalid string and is detected as such. There are many other forms of errors, like forgetting a matching > to every <, etc. Same if the error is in the string that represents a legend entry. For x/y labels, though:

set xlabel  `<i1>a</i0>b'
x=[1:10]
graph x,sin(x)

causes a crash:

 Unhandled unknown exception; terminating the application.

Need to improve error handling in the string parser called by graph.